Biochemical oxygen demand (BOD) is defined as ............ in ppm of `O_2`.

Options

A. (a) Required to sustain life
B. (b) The amount of oxygen required by bacteria to break down the organic matter present in a certain volume of a sample of water
C. (c) The amount of oxygen required by anaerobic bacteria to break down the inorganic matter present in a certain volume of a sample of water
D. (d) Required photochemical reaction to degrade waste

Answer

**Step-by-Step Solution:**

1. **Understanding BOD**: Biochemical Oxygen Demand (BOD) refers to the amount of oxygen that bacteria will consume while decomposing organic matter in a water sample. It is a critical parameter in assessing the quality of water.

2. **Analyzing the Options**:
- **Option A**: "Required to sustain life" - This is too vague and does not specifically relate to the decomposition of organic matter by bacteria.
- **Option B**: "Amount of oxygen required by bacteria to break down the organic matter present in a certain volume of sample of water" - This directly describes the process involved in BOD.
- **Option C**: "Amount of oxygen required by anaerobic bacteria to break down the inorganic matter present in a certain volume of sample of water" - This is incorrect because BOD specifically involves aerobic bacteria and organic matter, not anaerobic bacteria or inorganic matter.
- **Option D**: "Required photochemical to degrade waste" - This does not relate to the definition of BOD.

3. **Conclusion**: Based on the analysis, the correct definition of BOD is given in **Option B**.

**Final Answer**: The biochemical oxygen demand (BOD) is defined as the amount of oxygen required by bacteria to break down the organic matter present in a certain volume of sample of water, measured in ppm of O2.

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