If you are referring to organic evolution, then the correct answer would be choice a. Evolution is change in species over time. Since the environment is constantly changing, environmental pressures are placed on living species in terms of being able to adapt to their surroundings, find food, escape predation, etc. Those variations within a population, which are actually genetic mutations, may allow an individual to have an advantage. If that is the case, it will be selected for. If the individual passes along these variations, over time, more members of the species will also have these variations. Eventually, a new species may arise. This change in species over time is evolution.