Answer: c natural selection - smaller guppies became less visible to predators d selective breeding - predators select prey based on preferred traits.
In nature, predators prefer larger individuals. They will provide sufficient nourishment, not only to compensate for the used energy during the hunt, but also to sustain the individual for a period of time. Smaller individuals are hunted only if they are infants and thus vulnerable and easily caught.
Thorugh the introduction of the predator, being small became evolutionary advantageous. Thus, large guppy where preyed upon and did not succeed in reproducing. However, small guppy survived and reproduced successfully, thus passing on their genes. The next generation will be composed by the offspring of the smaller fish.