The answer is; C
This phenomenon is called bottleneck effect and drastically affects small populations in comparison to large ones. The mudslide wipes off most of the population B at the base of the hill base. Due to their small population, a large proportion of the population is lost and the pollution loses most of its alleles leaving a small proportion to continue the population succession. Its diversity is hence reduced as it repopulates. The larger population is not greatly affected because it covers much of the hillside.