The correct response is inbreeding.
In small populations, the gene pool is often reduced because inbreeding can occur more frequently, leading to a higher chance of offspring inheriting the same alleles from both parents. This reduces genetic diversity and can result in the prevalence of harmful traits or reduced fitness in the population. While bottleneck events and lack of suitable mates can also play a role in reducing gene pools, inbreeding is the most direct factor concerning the genetic makeup of small populations.