How do I know which attack a creature would use for an opportunity attack?

Question

Take for example a Ruffian CR 1/2 creature, he has a Multiattack Action of 2 Shortswords both with a reach of 5 ft so when something provokes an Opportunity Attack against it, it is obvious that the Ruffian will take one Shortsword attack on the provoking creature because he only has one type of attack.

But, what about an Adult Dragon with a more complicated Multiattack Action of 1 Bite, 10 ft range that deal 2d10 + Str; and 2 Claws, 5 ft range that deal 2d6 + Str? In addition to the Multiattack, the Dragon also has a Tail attack with a range of 15 ft and deals 2d8+Str.

So, how do I know which attack to use?

Does a creature provoke an Opportunity Attack when it moves 10 ft away? (i.e. Bite)
Or does it provoke an Opportunity Attack when it moves 5 ft away? (i.e. Claw)

Answer 1

PHB p.195

You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature.

MM p.11

MULTIATTACK

A creature that can make multiple attacks on its turn has the Multiattack ability. A creature can't use Multiattack when making an opportunity attack, which must be a single melee attack.

There are 2 possible interpretations:

1. A creature's reach is a singular thing defined by the reach of its furthest melee attack capability, or
2. Each and every melee attack capability grants the creature a reach; a creature has as many reaches as it has melee attack types.

Option 1

Your ruffian, whose longest melee reach is 5 feet (shortsword), if you move from within 5 feet to outside 5 feet without using the Disengage action, he can attack you with 1 shortsword if he has his reaction available.

For your dragon, whose longest melee reach is 15 feet, if you move from within 15 feet to outside 15 feet without using the Disengage action, he can attack you with 1 tail attack (the only one that will reach), if he has his reaction available.

Option 2

For the ruffian, this interpretation makes no difference.

For the dragon, it has 3 reaches - Claws 5 feet, Bite 10 feet and Tail 15 feet. If it has a reaction available and you move:

1. From 5 feet to 10 feet, it can attack with any one of Claws, Bite and Tail,
2. From 10 feet to 15 feet, it can attack with any one of Bite and Tail,
3. From 15 feet to 20 feet, it can attack with its Tail.

As the dragon only has 1 reaction per turn it can only elect to make one of these even if your movement triggers all 3.

Answer 2

A creature with several reach attacks can only make an opportunity attack with an attack for which the conditions to trigger an opportunity attack are met.

The best available authority on this issue is actually Jeremy Crawford's guidance on adjudicating opportunity attacks by a PC with the War Caster feat.

Bear with me here. One of War Caster's benefits is to let you cast a spell instead of making an opportunity attack. See PHB p. 170. Of course, many spells have a range greater than the standard 5 feet. So what happens when a PC with War Caster is wielding a 10-foot-reach weapon, like a polearm? Because of the PC's greater reach with the polearm, an opponent ordinarily would trigger an opportunity attack from the PC if it moved from 10 feet to 15 feet. So, if the PC also has War Caster, could she opt to cast a spell instead? Crawford says no:

Q: If I have the war caster feat and a reach weapon can I use a spell instead of the weapon if they move to 15 ft?

A: The intent is that any OA triggered because you're wielding a polearm is then made with that polearm.

In other words, the PC can't cast a spell instead of making an opportunity attack because the spell isn't what gives the PC reach. The polearm is what gives the PC reach. Thus, the polearm is the only attack method as to which the conditions to trigger an opportunity attack are satisfied by the target's leaving that reach.

Logically, there is no reason to treat any opportunity attack from any creature, whether monster or PC, any differently. If a creature has an attack method that offers a certain reach, it gets to make an opportunity attack with that attack method if and only if the target's movement triggered the opportunity attack at the reach of that attack method.

The adult dragon used as an example in the question has:

• a claw attack at ordinary 5-foot reach
• a bite attack at 10-foot reach
• a tail attack at 15-foot reach

If a PC moves from 5 feet to 10 feet, the only opportunity attack the dragon can make would be with the claw. The PC is not moving out of the dragon's reach for purposes of the bite or tail attacks.

If the PC moves from 10 feet to 15 feet, the only opportunity attack the dragon can make would be with the bite. The PC is not moving out of the dragon's reach for purposes of the claw or tail attacks. (And anyway the PC would be out of reach of the claw.)

If the PC moves beyond 15 feet, the only opportunity attack the dragon can make would be with the tail. The PC is not moving out of the dragon's reach for purposes of the claw or bite attacks. (And anyway the PC would be out of reach of both.)

Finally, just to complete the thought: imagine a creature with several attack methods that all have the same 10-foot reach, but only one that says it can be used to make an opportunity attack when a PC attempts to cast a spell. If a PC attempts to cast within 10 feet of the creature, we would not say the creature can make any kind of opportunity attack it wants just because the PC's action satisfied the conditions for one particular kind of opportunity attack. The opportunity attack would be made with the particular kind of attack that says it can be made under those conditions.

Reach, of any kind, should work no differently. It provides a set of conditions for making an opportunity attack. If an opponent does something to meet those conditions, the creature with reach gets to make the kind of opportunity attack for which the conditions are met, and nothing else.