The correct answer is the option (A) become extinct over time.
Genetic variation refers to the change in the genetic material of an organism. It helps the populations to adapt to the change in the environment.
A changing environment demands a change in the genetic material of the organisms in order to survive and reproduce. More variations in the genes leads to the production of the favorable traits helping the organisms to survive in the changing environmental conditions. These well adapted individuals produce more offspring. Whereas, the orgainsms that lack genetic variation tend to become extint as they are not able to cope up the environmental stress or changing environmental conditions.
Thus, the outcome for a species of fish that lacks the genetic variation necessary to adapt to a changing ocean environment is to become extinct over time.