Fossils show that ancestors of the giraffe had shorter necks than the modern giraffe. The evolution of the modern giraffe may be an example of .
A.
sexual selection
B.
directional selection
C.
disruptive selection
D.
stabilizing selection

B. Directional selection

Explanation:

When natural selection starts to choose one phenotypes that is an extreme (as in, a really small neck or a really long neck), this is an example of directional selection. It makes sense for giraffes to have evolved this way, because the longer the neck, the more ably they can eat food from high branches. The more food they could get, the more chance of reproduction, the more "evolutionarily fit" they are.

Stabilizing selection is when national selection picks average phenotypes (like a medium-sized neck). Disruptive selection is when the extremes are both picked (like all the medium-sized neck giraffes die, and two different species start to emerge). Sexual selection is picking a mate based on traits. Clearly, those answers don't work here.

b is the best answer for this question.