Translation:Qui est mort ?
It can mean either. Mourir is one of the few verbs that uses être rather than avoir as its auxiliary verb in the passé composé, so the phrase qui est mort ? is parallel to any other passé composé phrase like qui a mangé ? or qui a parlé ? or qui est parti ? That said, you could just as well interpret être as the main verb and mort as an adjective, so it could also be “Who is dead?”, yes.
But I assume this sentence is from a unit about using être in the passé composé and so they want you to translate it as the past-tense “Who died?”.
I didn't know there was a grouping of verbs that take avoir.
I think you have this backwards. Avoir is the default auxiliary verb for the passé composé. The verbs that use être are the exception.
There's a specific group of 16 verbs that use être as the passé composé auxiliary verb that is often referred to as the "Dr. and Mrs. Vandertramp" Verbs, since they're often arranged in the form of the acronym DRMRSVANDERTRAMP. The exact ordering of the verbs doesn't matter, but they are:
- Devenir "to become"
- Revenir "to come back"
- Mourir "to die"
- Retourner "to return"
- Sortir "to go out"
- Venir "to come"
- Aller "to go"
- Naître "to be born"
- Descendre "to go down/descend"
- Entrer "to come in/enter"
- Rentrer "to come back in/re-enter/to come back home"
- Tomber "to fall"
- Rester "to stay" (NOT "to rest" - beware of false cognates!)
- Arriver "to arrive, to happen"
- Monter "to go up [e.g. the staircase], to get in [e.g. the car], to get on [e.g. the bus]"
- Partir "to leave"
Some teachers add an extra P to the end for passer "to go by", as in Il est passé devant ma maison aujourd'hui "He passed by my house earlier today".
Just for extra fun, many of these also have extra meanings that do use avoir in the passé composé. For example, sortir uses être when it means "to go out" but avoir when it means "to take out" - so Il est sorti avec les valises (He went out with the luggage) but Il a sorti les valises (He took the luggage out).
Lastly, all reflexive and reciprocal verbs use être in the passé composé. There is no exception to this that I'm aware of.