Concept:Fertilizers in fishponds help increase the natural food available to fish by boosting plankton growth.
Explanation:Fish in ponds feed on tiny living organisms called plankton.
Fertilizers release essential minerals such as nitrogen and phosphorus into the water.
These minerals encourage plankton to multiply rapidly.
The increased plankton population then serves as food for the fish.
So fertilizers are not used as bait, direct fish food, or water purifier.
Their main role is to supply nutrients for plankton.
Answer:C. nutrients for plankton