1. 
tadpole     frog     amphibian
lamb     sheep ?

A. animal
B. wool
C. farm
D. mammal

Answer: Option D

Explanation:

The tadpole is a young frog; frogs are amphibians. The lamb is a young sheep; sheep are mammals. Animal (choice a) is incorrect because it is too large a grouping: Animals include insects, birds, mammals, reptiles, and amphibians. Choices b and c are incorrect because they are not part of the progression.