The Serengeti is the world's largest

a. desert.

b. mountain range.

c. savanna.

d. lake.

Serengeti is a land region (so we can reject the answer "lake") and it's relatively flat (so we can reject the answer "mountain rage"). It supports a great amount of animal life - so we can also reject a "desert".

It's a savanna - a grassland with occasional trees.