Final answer:
The best description of "survival of the fittest" is that it correlates with the concept of natural selection, but was coined by Herbert Spencer for social theories, not by Darwin. The term relates to reproductive success and adaptation over time.
Step-by-step explanation:
The statement that BEST describes the phrase "survival of the fittest" is that it is another way of saying "natural selection." However, it was Herbert Spencer, not Charles Darwin, who first coined the term to describe the outcome of natural selection in a way that he applied to social theory, an ideology that became known as social Darwinism. This concept does imply that organisms with traits best suited for their environment are more likely to survive and reproduce, leading to evolutionary change, but it was misused historically in social contexts to justify things like class distinctions and racial superiority claims.
The phrase does not merely refer to changes in species over time, nor does it originate from Darwin to explain evolutionary theory, yet it is closely associated with the mechanisms of natural selection which Darwin and Alfred Russel Wallace independently conceived. It's important to note that fitness in this context refers to an organism's reproductive success and not necessarily to their strength or other common misconceptions about the term.