The correct answer is D. High 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 sewage, industrial waste, or agricultural runoff. This organic matter can support the growth of bacteria and other microorganisms, which can use up the oxygen in the water. This can lead to problems for fish and other aquatic life, as well as for people who use the water for drinking or recreation.
Option A is incorrect because a high BOD indicates that there is a lot of organic matter in the water, not that the water is pure.
Option B is incorrect because microbial action is necessary to break down organic matter and cause a high BOD.
Option C is incorrect because a low level of microbial pollution would result in a low BOD.