A river with an inflow of domestic sewage rich in organic waste may result in:

The correct option is D Death of fish due to lack of oxygen.
When domestic sewage which is rich in organic waste enters the river the BOD (Biological oxygen demand ) of the river water increases.
BOD is the amount of dissolved oxygen used by microorganisms in the biological process of metabolizing organic matter in water. The more organic matter there is the greater the BOD; and the greater the BOD, the lower the amount of dissolved oxygen available for higher animals such as fishes. This leads to the death of the fish in the river.

Algal blooms in the river are due to the inflow of excessive nutrients.