Final answer:
Natural selection explains how the process described in Part A changed the frequency of alleles in Darwin's finches.
Step-by-step explanation:
The process described in Part A refers to natural selection. Natural selection is the process in which better-adapted organisms are able to survive and reproduce. In the case of Darwin's finches and their changing beak size, the frequency of alleles changed because finches with traits better suited for survival were able to reproduce and pass these traits to their offspring. This resulted in an increase in the frequency of alleles that conferred beneficial traits.