Natural selection is the process in which some organisms that show better adaptation to the environment are more likely to produce an offspring. In the case of the exercise, the answer is:
b. Some tuna live in the Pacific Ocean. They are fast, so they are likely to survive and pass on their genes.
It means that as they are better adapted to that environment due to the fact they are fast swimmers, they are likely not only to survive, but to pass their genes and produce offsprings.
