"Which animal is the strongest?"
Translation:Które zwierzę jest najsilniejsze?
the answer to
które zwierzę ? animal nr 5
jakie zwierzę ? the pink one
but both are good ( and google has 40x more hits with "jakie")
I don't think so. I can't say why, but "mocny" is just a lot less commonly used than "silny" and it has its collocations, outside of which it just looks strange.
Somehow that would make more sense to me if you were talking about Pokemon, not real-world animals. Although I'd still go with "najsilniejszy" even then.