The correct answer is D
The process of evolution presents a change in population genetics over periods of time. Therefore, it's important to remember that we can only observe the evolution by looking at the level of the population, not on the level of an individual organism.
Evolution works trough 4 evolutionary mechanisms that change the frequency of an allele in a population, leading to evolutionary change of that population and later even species. These mechanisms are : mutations, genetic drift, gene flow and natural selection.