The correct answer is: C. Both are correct because genetic variation can occur immediately or long period of time.
Genetic diversity is the sum of all of genetic characteristics in the genetic makeup of a species and it helps population to adapt to new environment conditions. The more variation is in population the more adaptive to environment it will be.
Genetic variation can occur as a result of random mutation, which happens immediately.
On the other hand, continual process of natural selection leads to genetic diversity over a long period of time by favoring the strong and eliminate the weak genes.
Genetic drift and gene flow (migration) also have impact on genetic diversity.