BOD (biological oxygen demand) is

Options

A. the amount of oxygen required by bacteria to break down the organic matter of a sample of water
B. the amount of chemicals required to break down the organic matter of a sample of water
C. the amount of phosphate required to oxidise the organic matter of a sample of water
D. the amount of organic matter present in the given sample of water.

Explanation

### Step-by-Step Solution:

1. **Definition of BOD**: Biological Oxygen Demand (BOD) is defined as the amount of dissolved oxygen that microorganisms, such as bacteria, require to decompose organic matter in a water sample.

2. **Role of Microorganisms**: Microorganisms in water bodies utilize oxygen to oxidize organic wastes. These wastes can include dead organisms, human feces, and other organic materials that enter the water.

3. **Oxygen Consumption**: As microorganisms break down these organic materials, they consume dissolved oxygen from the water. This consumption leads to a decrease in the oxygen levels available in the water body.

4. **Indicator of Organic Waste**: A higher BOD value indicates a greater amount of organic waste present in the water. Therefore, BOD serves as an important indicator of water quality.

5. **Options Analysis**:
- Option 1: The amount of oxygen required by bacteria to break down the organic matter of a sample of water - **Correct**.
- Option 2: The amount of chemical required to break down the organic matter of a sample of water - **Incorrect**.
- Option 3: The amount of phosphate required to oxidize the organic matter of a sample of water - **Incorrect**.
- Option 4: The amount of organic matter present in a given sample of water - **Incorrect**.

6. **Conclusion**: The correct answer is that BOD is the amount of oxygen required by bacteria to break down the organic matter of a sample of water.