A gazelle is born with slightly longer legs which allow it to run faster than the other gazelles in the herd. This adaptation came may have come about because

Athe gazelle needed to be able to run faster.
Bthe gazelle's parents were not able to outrun a cheetah.
Cthere was a mutation in the gazelle's genetic code.
Dthe gazelle did not get enough nutritious food after it was born.

Verified Answer :

Because there was a mutation in the gazelle's genetic code. It will be nice for the gazelle to run faster, but since the gazelle was born with longer legs, we know it's just a random mutation. The gazelle's parents have little to do with it, and the nutrition of the food it eats after it's born is irrelevant to how long its legs are at birth.

Otherwise known as answer: C.