Genetic diversity can be defined as the process of the variation in the genetic information of the organisms occurring naturally over time. This process of genetic variation leads to speciation, which results ti the evolution of the new species. Genetic variation can occur over a long period of time, or it can happen suddenly.
Over the time, by the process of natural selection, the strong genes get selected and passed on while the irrelevant one are eliminated. And over a long period of time, this continual process of natural selection leads to genetic diversity. Random mating, and fertilization also leads to the genetic diversity in a population.
On the other hand, the events, such as mutation, often leads to the sudden genetic diversity.
Hence, Sarah, and John both are correct. The correct answer is 'option C'.