Answer: A. Natural selection
Step-by-step explanation:
Natural selection is a mechanism of differential survival and reproduction of the members of the population of a species those have better traits. These traits help them to adapt, survive and reproduce in adverse environment.
Giraffe growing long neck is the example of natural selection. The long neck is the phenotypic adaptation which develop in order to fetch leaves from branches of long trees in the scarcity of grass and small plants in the ground.