Can i have the answer?

Prokaryotes are single cell organism. Prokaryotes were the most earliest organism and they lived and evolved all alone on earth for 2 billion years. They have continued to adapt and flourish on an evolving earth and in turn they helped the earth to change it.  Prokaryotes, the simple creatures fed on carbon compounds that were accumulating in Earth's early oceans. Slowly, other organisms evolved that used the Sun's energy, along with compounds such as sulfides, to generate their own energy.

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