The correct answer is C. Oxygen.
Eutrophication is the process by which a water body becomes enriched with nutrients, particularly phosphorus and nitrogen. This can lead to an increase in the growth of algae and other aquatic plants, which can block sunlight and prevent other plants from growing. The decomposition of these plants can also deplete the water of oxygen, which can kill fish and other aquatic life.
Option A is incorrect because food is not the main factor that limits the growth of fish in eutrophic water bodies. The availability of oxygen is more important.
Option B is incorrect because light is not the main factor that limits the growth of fish in eutrophic water bodies. The availability of oxygen is more important.
Option D is incorrect because essential minerals are not the main factor that limits the growth of fish in eutrophic water bodies. The availability of oxygen is more important.