Similar to this question about using Deflect Missiles against unusual projectiles, but way simpler.
The monk's Deflect Missiles feature states:
If you catch a missile in this way, you can spend 1 ki point to make a ranged attack with the weapon or piece of ammunition you just caught, as part of the same reaction. You make this attack with proficiency, regardless of your weapon proficiencies, and the missile counts as a monk weapon for the attack, which has a normal range of 20 feet and a long range of 60 feet.
Assume a Monk is hit by a Crossbow Bolt and manages to nullify the damage. The same Bolt can be used by three different Crossbows (Hand, Light or Heavy), to deal 1d6, 1d8 or 1d10.
Does the damage dealt by the reaction attack from Deflect Missiles depend on the weapon that was used to attack the Monk in the first place? That seems very weird, since the difference is pretty much only the speed that the crossbow fires the bolt - which is meaningless once the Monk has stopped the shot.
In particular, the text states that the missile counts as a monk weapon for the attack. Does it mean we should use the damage from the monk's Martial Arts feature?
You can roll a d4 in place of the normal damage of your unarmed strike or monk weapon. This die changes as you gain monk levels, as shown in the Martial Arts column of the Monk table.
The problem here is that it says you can replace the weapon damage for the Monk Dice (this is how I call the dice from the column), but not necessarily has to.
So, what damage dice do I roll if the reaction attack from Deflect Missiles hits?