Answer:
D
Step-by-step explanation:
The only way for giraffes to evolve over time towards having long necks would be to have genetic differences that were gradually favored over time. As in, the giraffes that were genetically disposed to have necks just a bit longer eventually survived longer, because they could eat high leaves, while those with short necks died, and the long-necked giraffes reproduced more, which increased the allele frequencies of long necks in the species over time.
Never, never say A or B!! This is known as Lamarck's theory of inheritance of acquired characteristics, and it's simply wrong.
C is wrong, because the mutation must occur in the original zygote.