What phenomenon results from the runoff of fertilizers from agricultural and residential areas causing the aquatic algae in bodies of water to grow prolifically?

Eutrophication is what the phenomenon called.  Eutrophication This process induces growth of plants and algae and due to the biomass load, may result in oxygen depletion of the water body. It is a choking of rivers, lakes and other waterways by excess algae growth which has been stimulated by fertilizers or sewage.

what's the rest of the question

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."