[susan53]

This is difficult to answer out of context. Imagining a text which described the habits of three different animals - say, lions, horses and seals and explained that lions eat meat, horses hay and seals fish - then the best question would be Which animal eats meat? or Which of the three animals eats meat?

If the question was general rather than restricted to a limited choice (as above, where I have three possibilities only), then you could say : What animals eat meat? - the answer being lions, tigers, cats, dogs, hamsters and all other carnivores and omnivores.

Who eats meat? would only be used if the lion was being anthropomorphised - ie given human qualities. This might happen in a book for small children : Now at the zoo we can see Larry the lion, Harry the horse and Sallie the seal. One of them eats meat, one eats hay, and one eats fish. Who eats fish?

What eats meat? is very unlikely to be used at all.