Question

Biped quadruped ostrich

Answer 1

"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