Please submit your concern

Explanation:

First sentence of the last paragraph says “as the population continued to expand and the islands kept on receiving immigrants from the original population, the calls in all locations would have eventually regressed to the average pulse rate of the founder colony”. 

This is a simple concept of average. Now the question is which of the following could have ensured (it means it is asking us for a hypothetical situation) that male northern elephant seals dialects did not disappear. 

It disappeared because the average changed because of migrant seals. As more and more seals came, the average regressed to “the pulse rate of the founder colony”. To make the situation opposite, the call tempo of the individual immigrant seal should have changed to match the average tempo of resident male seals of the “host colony.” 

If option (a) had happened, the male northern seals dialect would not have disappeared. 

Option (c) is the exact opposite of option (a). We must remember here that the islands kept on receiving immigrants from the original population, and the average pulse rate changed to match the founder colony, not the host colony. That’s why the host colony’s dialects disappeared. 

Had option A been true, this would not have happened.

Hence option (a).