The gene pools of small populations are often reduced compared to those of larger populations primarily due to inbreeding. In small populations, there are fewer individuals to mate with, which increases the likelihood of mating between closely related individuals. This can lead to a decrease in genetic diversity, as the gene pool becomes more homogeneous.
While bottleneck events can also contribute to reduced gene pools (specifically by sharply decreasing population size and genetic diversity), inbreeding is a more direct reason for the limited genetic variation observed in small populations.
Therefore, the best response from the options provided is inbreeding.