Population replacement among species of organisms in a given habitat under competition is due to

Population replacement among species in a given habitat under competition is a core outcome of natural selection. Natural selection is the process where organisms better adapted to their environment (including competition for limited resources) are more likely to survive, reproduce, and pass on their advantageous traits to the next generation, eventually leading to the replacement of less-adapted species or populations over time. The concept of "survival of the fittest" is often associated with natural selection, but natural selection is the formal, broader mechanism of evolution that explains this phenomenon. 

A. survival of the fittest: While related, "survival of the fittest" is a phrase used to describe the result (the fittest survive), but natural selection is the actual mechanism or law that causes this differential survival and reproduction, leading to population changes.

B. climax community: is the stable, final stage of ecological succession in an area, where the species composition remains relatively constant over time because they are well-adapted to the local conditions, not the cause of population replacement during the competitive process itself.

D. dominant population: is one that outcompetes others, but this dominance is a result of the underlying process (natural selection), not the fundamental mechanism explaining the replacement across species generally