Biped quadruped ostrich

"Biped" means using 2 legs and "quadruped" means using 4 legs.

So we have: using 2 legs: using 4 legs.

Ostriches do in face use 2 legs.

Now we have to choose the 4-legged animal from the set of:

snake human kangaroo cat

And the answer is cat!

Snakes don't have legs, and humans and Kangaroos have 2 legs each