a) The environment.
Explanation: Natural selection is one of the forces of evolution. Natural selection occurs when individuals in a population have a different reproductive success in certain environment. For example, imagine a mosquitoes population in a insecticide-free envrionment, the non-resistent mosquitoes will be more succesfull that the resistant ones. But when there's insecticide in the environment, the resistent mosquitoes will be more successfull that the non-resistent ones, because they die.