Final answer:
Differential reproduction leads to natural selection as it results in favorable traits becoming more common in a population. Random mating, high gene flow, and small population size can influence genetic makeup, but they do not constitute natural selection by themselves.
Step-by-step explanation:
The situation that will lead to natural selection is c) Differential reproduction. This is because natural selection involves organisms with more favorable traits reproducing more successfully than others, leading to those traits becoming more common in the population over time. Situations like random mating, high gene flow, and small population size can affect the genetic structure of a population, but they do not necessarily result in the non-random survival and reproduction of individuals based on their inherited traits, which is the key mechanism of natural selection.
Random mating, high gene flow, and small population size are all factors that can influence the genetic makeup of a population. For example, random mating can alter allele frequencies due to chance pairings, high gene flow can introduce new alleles into a population or equalize allele frequencies between populations, and small population size can lead to genetic drift, where random events can cause significant changes in allele frequencies.