Answer:
c. Reproductive isolation
Step-by-step explanation:
Reproductive isolation is a process whereby organisms of a species are unable to mate and reproduce successfully as a result of barriers such as physical barrier or being isolated from each other.
When a subpopulation become isolated from its original population, the subpopulation would most likely not carry along all the copy of the genes in the gene pool the main population. Over time this subpopulation might be acted upon by evolutionary forces which would make the allele frequency to change and become different from those of its original population, as gene flow is impossible between both isolated populations. This leads to diversity as both populations over time would differ from each other as they have become reproductively isolated.
The most likely explanation for the difference in allele frequencies is reproductive isolation.