What reactions are available to an artificer's homunculus servant?

Question

The description of the artificer's homunculus servant states:

In combat, the homunculus shares your initiative count, but it takes its turn immediately after yours. It can move and use its reaction on its own, but the only action it takes on its turn is the Dodge action, unless you take a bonus action on your turn to command it to take the action in its stat block or the Dash, Disengage, Help, Hide, or Search action.

The homunculus has a limited number of action options in combat, and most require the artificer to use their bonus action on their own turn. This (closed) question lists the reactions that are generally available in the game, but the homunculus can't have feats, use the Ready action, or take an Opportunity Attack.

Yet the rules specifically go out of their way to mention that the homunculus can "use its reaction on its own".

Question: what reactions can it use?

Answer 1

Channel Magic, external sources, and the optional facing rule

It is true that the most common reactions are unavailable to the homunculus servant. However, it still has some options.

Channel Magic is a reaction listed in the servant's stat block, functioning similarly to the spell delivery mechanism of find familiar. This is presumably the biggest reason reaction usage is specifically called out.

Channel Magic. The homunculus delivers a spell you cast that has a range of touch. The homunculus must be within 120 feet of you.

Eberron, Rising from the Last War p.62

Outside of the option inherently available to the homunculus servant, some magical effects and mundane situation call for the use of reactions. Spells, such as dissonant whispers and power word heal, can cause the target to use its reaction either voluntarily or forcefully. It is certainly possible within the rules for monster abilities, traps, or other situations to have similar stipulations. One example is the Path of Blades complex trap from Xanathar's Guide to Everything (p.118)

Magic items can grant reactions, as well. If you wanted to make your homunculus the envy of your friends by attuning it to an Ioun Stone of Greater Absorption (and your GM is permissive), well, that's an option you have.

Another niche option comes from the optional facing rule in the DMG (p.252), which allows creatures to change their facing as a reaction when other creatures move. This may or may not matter to the homunculus, but again, it is an option you have.