Final answer:
Mutation is the change in DNA's chemical composition and is the key source of genetic variation, serving as a cornerstone for evolutionary processes.
Step-by-step explanation:
The correct answer to the question is A) Mutation. Mutation is the change of the chemical composition of DNA and is the source of genetic variation which leads to evolution. Every individual has a number of mutations in their DNA. These mutations serve as the ultimate source of new genetic material, or alleles, for a species. While most of these changes have no impact, some can be beneficial or harmful.
They become particularly important for evolution because harmful mutations tend to be weeded out through natural selection, and occasionally, beneficial mutations can spread through a population, increasing the organism's fitness and likelihood of survival and reproduction. Mutations are random events, but their frequency can be influenced by external factors such as ultraviolet light or chemical agents. Overall, mutation provides the raw material on which other forces of evolution, such as natural selection, act.