B is basically a test to see if you're racist or not.
A, however is another one of those "what answer do they want me to give?!" ones. Assuming they don't mean a linguistic difference ("Fish is the only one for which the plural is the same as the singular"), but a physiological one, there are just so many. It's basically asking what makes fishes, mammals, reptiles, frogs and birds different from each other.
Fish Only one with fins. Only one without lungs.
Rat Only one with fur. Only one that has babies. Only one that carries the Black Death. Only one that Pied Piper can charm.
Snake Only one that sheds its skin. Only one able to eat something bigger than its head. Only one Indiana Jones is scared of.
Frog Only one without a tail. Only one to undergo metamorphosis. Only one that can jump.
Hen Only one with feathers. Only one with a beak. Only one that can't live in water. Only one that's only female.
I'm going with Kanye West, becasue he's a singer and the rest are actors.