"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