BOD (Biochemical Oxygen Demand) is the correct answer.

BOD is a measure of the amount of oxygen required by microorganisms to break down organic matter in sewage. It is an important parameter in wastewater treatment and is used to assess the level of organic pollution in water bodies.

Here is a detailed explanation of why BOD is the correct answer:

1. Biochemical Oxygen Demand (BOD):
- BOD is a measure of the amount of dissolved oxygen needed by aerobic microorganisms to decompose the organic matter in sewage.
- It is expressed in terms of milligrams of oxygen consumed per liter of water (mg/L) during a specific period of time, usually 5 days (BOD5).
- BOD is an indirect measure of the organic pollution level in water bodies, as higher BOD values indicate higher levels of organic matter and pollution.

2. Organic matter in sewage:
- Sewage contains various organic substances, such as human waste, food particles, detergents, and industrial effluents.
- These organic substances are biodegradable, meaning they can be broken down by microorganisms through the process of aerobic decomposition.
- During this process, microorganisms consume oxygen to metabolize the organic matter and convert it into simpler compounds like carbon dioxide, water, and microbial biomass.

3. Oxygen requirement for organic matter oxidation:
- The oxidation of organic matter requires a significant amount of oxygen to support the growth and metabolism of microorganisms.
- The higher the concentration of organic matter in sewage, the higher the demand for oxygen.
- BOD reflects this oxygen requirement and indicates the level of organic pollution.

4. Turbidity:
- Turbidity refers to the cloudiness or haziness of a fluid caused by suspended particles.
- It is not directly related to the amount of oxygen required to oxidize organic matter in sewage.
- Turbidity can be caused by various factors, including inorganic particles, sediments, and microorganisms, but it does not provide information about the organic pollution level.

5. COD (Chemical Oxygen Demand) and DO (Dissolved Oxygen):
- COD is another parameter used to measure the amount of oxygen required to oxidize organic and inorganic matter in sewage.
- DO, on the other hand, measures the concentration of dissolved oxygen in water.
- While both COD and DO are important in wastewater treatment, they do not specifically quantify the oxygen demand for organic matter oxidation like BOD does.

In conclusion, BOD is the correct answer because it specifically measures the amount of oxygen required to oxidize only organic matter in sewage. It provides valuable information about the organic pollution level and helps in evaluating the effectiveness of wastewater treatment processes.