Hello! I would agree with you, I think Mutation is the correct answer. Let's go through why:
A) Mutation: mutations are random changes to DNA. These introduce variations of different alleles, and therefore introduce more genetic diversity. For example, the mutation that causes blue eyes occurred many, many years ago, and before the mutation occurred, most people had brown eyes. You add another eye color, you add genetic diversity!
B) Genetic drift: Genetic drift is something that occurs due to a random change in the frequency of alleles in a population. For example, if you have both brown and black dogs, and just by chance they have only black puppies, this increases the number of alleles of the black fur allele in the population, but not through any kind of advantage, it was random.
C) Migration: Migration is the movement of animals from one habitat to another over the year (like how geese migrate to the south for the winter). It doesn't change anything about the genetic diversity of the species.
D) Selection: Selection is the process where more advantageous alleles out-compete less helpful ones, and therefore change in frequency. For example, if a population of tortoises on an island eat plants that are tall, and the long-necked turtles are more successful in staying alive and producing offspring compared to short-neck turtles, we would say that is an example of selection. This doesn't increase the number of alleles in a population, and, if one allele is super un-helpful or harmful to an animal population, it can actually lead to the decrease in the number of alleles in a population when animals with that allele aren't able to survive and reproduce to pass the less helpful gene on.
I hope this was helpful, if you're looking for any more help with biology feel free to reach out to me regarding a tutoring session!