Answer:
The right answer to this question is: Adaptation
Step-by-step explanation:
Adaptation is based on Darwin's theory, and it's not that simple. The adaptation of a species to fit an environment is almost always through random mutation, and these mutations can or cannot help the organism, it is completely random, but when it is good for them, this mutation stays on this population until it's not needed anymore.
Darwin's theory is called Natural Selection, but nowadays, this theory has changed a lot with time, and genetics can now contribute to the explanation.