Correct choice is (a) Algal blooms
Explanation: Phosphorus in water from runoff lands, or from leaching through soil from fertilizers and sewage discharges where the phosphorus content is too high, it can trigger algal blooms resulting in eutrophication which would eventually block the sunlight from reaching lower waters leading to the death of the plants, plants, aquatic animals do not block the sunlight, whereas, algae helps in photosynthesis.