Answer:
Option A
Step-by-step explanation:
In this case the two species of squirrels have evolved when their combined population got divided into parts and was separated as a cause of natural calamity i.e Colorado River's erosion of the Grand Canyon, thereby forced to live in two different geographies and hence evolve differently from each other. The two geographies mentioned above remain disconnected by any natural barrier thereby restricting successful mating of the two population groups of squirrels.
hence, option A is correct