Answer:
B. This situation represents isolation by vicariance
Step-by-step explanation:
Vicariance is a process by which the large geographical area is distributed in a small area by the formation of a barrier. This process distributes the large population in into small population due to the formation of the barrier which does not allow the gene flow.
In the given question, the ants live on the island of the river and the formation of a dam increases the water level during the flood. Only the highest point of the island appears which leads to the separation of the population of the ants. This is the case of the Vicariance.
Thus, Option-B is correct.