Answer: The correct answer is -
A) gene flow.
Step-by-step explanation:
Gene flow can be described as the process of transfer of genetic variation from one population to another population so that the sum total of all genetic diversity (gene pool) of the given population is altered.
It can occur through various means. For example - bird migration from one habitat to another habitat, which results in the change in gene pool.
Thus, option A) is the right answer.