Answer: D. no natural selection
Step-by-step explanation:
Fitness refers to the passing down of genetic make up based on the environmental requirements for reproduction and survival.
Answer choice A is incorrect, as large population, refers to the fact the more individuals in the population, the smaller effect of genetic drift. Genetic drift is a mechanism of evolution in which allele frequencies of a population change over generations due to chance (sampling error).
Answer choice B is incorrect, as random mating, refers to each individuals in a population have the same chance of passing on its alleles. An example of random mating includes: lions with darker fur color have the same chance to reproduce as lions with a lighter fur color.
Answer choice C is incorrect, as no mutations, refers to no changes to genes, new alleles are not introduced into the populations gene pool.
D is correct, no natural selection is when no phenotype can have a selective advantage over another - all individuals have equal fitness. And that correlates with the fitness being the same among all individuals in the population.