Final answer:
The differences between humans and chimpanzees, despite high genetic similarity, are primarily due to genetic mutations, which along with environmental factors and natural selection, have driven their evolutionary divergence.
Step-by-step explanation:
Though humans and chimpanzees share a high degree of genetic similarity, the differences between the two species can be accounted for by a combination of factors. Genetic mutations, natural selection, and environmental factors all play significant roles in driving evolutionary changes. Over time, these factors have led to the divergence of our species from a common ancestor.
Genetic mutations create new variations in a population's gene pool. When these mutations are beneficial, natural selection can increase the frequency of those advantageous alleles in the population. Environmental factors also influence the evolutionary paths of species by creating different pressures that favor certain traits. This process led to significant differences in anatomy, such as brain size and skeletal structure, even though our fundamental genetic makeup is quite similar.
Answering the multiple-choice question at hand, the best explanation for the differences observed despite the high genetic similarity would be: C. Genetic mutations.