Which Sequence of Events Describes Natural Selection

Which sequence of events describes natural selection?

A) A population of a mammal species varies in the length of their legs; water levels rise and the habitat becomes swampy; the individuals with shorter legs are better able to swim to food; the population evolves to become shorter-legged.
B) A population of a mammal species lives near the water; water levels rise and the habitat becomes swampy; the animals have trouble moving in the water; the population changes to a shorter-legged type so they can move more easily in the water.
C) Water levels rise, and a habitat becomes swampy; a population of a mammal species on high ground observes the change; individuals with shorter legs migrate to the area.
D) A population of a mammal species lives near the water; water levels rise and the habitat becomes swampy; the act of swimming shortens the legs of the mammals; the offspring of the mammal species have shorter legs.