Answer:
The best option here is B)
To reconstruct the irrigation system so that water from the river is allowed to enter the ecosystem.
Step-by-step explanation:
Without water, there is no life. It doesn't matter how nutrient-rich the place becomes.
Sewage run-off water is completely out of question. Sewage at this point will serve to increase the richness of the area. Besides, sewage contains various other compounds such as methane which may not be suitable for aquatic life.
The best next step is to restore water into the region then gradually re-enrich it's nutritional potential. As the eco-system becomes more suitable for Plankton other higher forms of aquatic life will flow through from the river into the eco-system.
Planktons are a diverse group of organisms which live in large water bodies and can not swim against a current. The plankton comprising individual species are called plankers. Both smaller and larger aquatic species, such as bivalves, fish, and whales, are supplied a vital source of food.
Cheers!