Final answer:
Natural selection is the process that contributes to the change in frequency of favorable traits over time.
Step-by-step explanation:
The correct answer is a) Natural selection.
Over time, the change in the frequency of favorable traits contributes to the process of natural selection, which is an important concept when it comes to both the unity and diversity we observe in all living things.
Natural selection causes evolution of a species as these beneficial traits become more common within a population. It allows for the adaptation and survival of individuals with favorable traits, leading to changes in the allele frequencies over time.