Answer: it will be A..
Explanation: Gene flow is the transfer of genes across two populations. This happens if there is mating between the two populations. However, this can be curtailed either by allopatry (due to geographical isolation) or sympatry (due to behavioral isolation) which results in reproductive isolation between the two populations. If the two population were of the same species, then the reproductive isolation can result in divergent speciation of the two populations.