If a zebra migrates to join a different herd of zebras is that mutation gene flow genetic drift or natural selection?

The correct answer would be gene flow.

Explanation:

Gene flow  is explained as the transfer of genes from one population to another so there is an increase in their frequency of genes.

It is also called migration. The addition or loss of people can affect the frequencies of the gene pool even if no other evolutionary mechanisms working at the time.

Thus, the correct answer would be gene flow.