The correct answer is C. intense level of microbial pollution.
Biological oxygen demand (BOD) is a measure of the amount of oxygen that is used by microorganisms to break down organic matter in water. A high BOD indicates that there is a lot of organic matter in the water, which can be caused by microbial pollution. Microbial pollution can come from a variety of sources, including sewage, industrial waste, and agricultural runoff. When there is a lot of microbial pollution in water, it can cause problems for fish and other aquatic life, as well as for people who use the water for drinking or recreation.
Absence of microbial pollution would have a low BOD, as there would be little or no organic matter in the water for microorganisms to break down. Moderate microbial pollution would have a moderate BOD, as there would be a moderate amount of organic matter in the water.
Therefore, the correct answer is C. intense level of microbial pollution.