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. 

Aregion is an area of land that has common features. these features can be natural, such as climate or landscape. they can also be artificial, such as language or religion. the xinjiang uyghur autonomous region, above, is an area of china inhabited by uyghurs, a muslim group with ethnic ties to turkish people.