Darwin's Theory of Evolution by Natural Selection.
Darwin divided this theory into multiple parts but overall it fits into Darwin's Theory of Evolution by Natural Selection, so that would be your answer.
In case you're wondering about the others:
1. Organisms produce more offspring than actually survive.
2. Every organism must struggle to survive.
3. There is variation within a species.
4. Some variations allow members of a species to survive and reproduce better than others.
5. Organisms that survive and reproduce pass their traits to their offspring, and the helpful traits gradually appear in more and more of the population.
(these are brief explanations, they go into more detail)
Good luck!
