What does a high value of BOD(Biochemical Oxygen Demand) indicate?

The correct option is "that water is highly polluted".

Understanding Biochemical Oxygen Demand (BOD):
Biochemical Oxygen Demand (BOD) is a chemical procedure used to determine the amount of dissolved oxygen needed by aerobic biological organisms to break down organic material present in a given water sample at certain temperature over a specific time period.

Why a High BOD Indicates High Pollution:
When water bodies receive large amounts of organic waste (such as sewage, agricultural runoff, or industrial waste), the population of decomposers and microorganisms (like bacteria) increases rapidly to consume this organic matter.
These microbes require oxygen for cellular respiration as they decompose the organic waste. Therefore, a larger amount of organic pollution leads to a higher rate of consumption of dissolved oxygen by these microbes, resulting in a higher BOD value.
Consequently, a high BOD value directly reflects that the water contains a large amount of biodegradable organic matter, indicating that the water is highly polluted.