The correct option is D Natural selection
The phrase “The survival of fittest” was first used by a British philosopher Herbert Spencer after reading Darwins ‘On the origin of species". He used the phrase as a synonym for natural selection.
The continued existence of organisms that are best adapted to the environment that enable them to reproduce is called the survival of the fittest; this in the paper of Darwin was explained as natural selection, infact, the main idea of the book, “On the origin of species” is that species evolve very slowly over a long period of time through the process of natural selection.
Evolution is a process that occurs over time in a population for their survival in the changing environment. The existing population has variations and variation leads to evolution. The variations with survival value are passed on to the following generations, so over generations species adapt and become better fitted with their environment.