Question

Biped : quadruped :: ostrich : snake human kangaroo cat

Answer 1

A biped is a being that walks on two legs. A quadruped is a being that walks on four legs. As you already know, an ostrich is a bird that walks on two legs (it is a biped). So in order to complete the analogy, you need to choose an animal that walks on four legs. The correct answer is cat. 
Snakes have no legs, and humans and kangaroos are bipeds. 

Answer 2

A biped is an animal with two legs
A quadruped is an animal with four legs

An example of a biped is an ostrich

From the choices:
snake - no legs
human - two legs
kangaroo - two legs
cat - four legs

Cat is an example of a quadruped