Final answer:
Drift is most influential in small populations where random events can have a significant impact on the gene pool, while selection is stronger in large populations where genetic diversity provides more material for advantageous traits to be selected.
Step-by-step explanation:
The correct answer to the question is: Drift is most powerful when population sizes are small; but selection is more powerful when they are large.
The subject of this question is evolutionary biology, particularly genetic drift and natural selection. Genetic drift is a mechanism of evolution that refers to random fluctuations in the frequencies of alleles in a population that occur due to chance events. This is especially pronounced in small populations where, if one individual dies before reproducing, a significant portion of the gene pool is lost. For example, if one out of ten individuals in a population dies prematurely, that's 10% of the gene pool gone. Conversely, in a large population, the death of one individual would likely only constitute a loss of a small fraction of the gene pool, thus less affecting the population's genetic structure.
Natural selection, on the other hand, acts on heritable traits that affect reproductive success and is more powerful in larger populations because there is more variation for selection to act upon; advantageous traits are more likely to be present and proliferate.