Final answer:
The best change that might benefit jellyfish is an increase in plankton, since it would amplify their food source without heightening competition or predation. Therefore , the correct answer options is d)
Step-by-step explanation:
The question pertains to the interaction among marine species, specifically jellyfish, anchovies, tuna, and swordfish, and their relation to the availability of food sources such as plankton.
Since jellyfish compete with anchovies for plankton, an increase in plankton would benefit jellyfish as it would enhance the food source for both species without directly increasing the competition. Conversely, an increase in predators like swordfish or tuna, which prey on jellyfish, would be detrimental.
Therefore, the change that might benefit jellyfish the most is Option D: an increase in plankton.