Answer:
The correct answer is - a. biological action.
Explanation:
Biological action depends on microbes such as bacteria, nematodes, or other small organisms to treat and break down organic or biodegradable pollutants or wastes through the water process.
Water bodies have various types of pollutants including biologically degradable garbage, wastes, and partially digested foods. In some of the lakes and ponds pathogenic organisms, heavy metals, and toxins also found that are treated by these microbes.
