Select the mismatched pair.

Options

A. Potable water - Water that is free from impurities and microorganisms.
B. BOD- The amount of oxygen required to oxidise all the reducing substances present in water.
C. Oil spills – Accidental discharges of petroleum from oil tankers and oil refineries.
D. Eutrophication - Enrichment of water by nutrients which leads to excessive plant growth and depletion of dissolved oxygen.

Explanation

To solve the question of selecting the mismatched pair, we will analyze each provided pair one by one.

### Step-by-Step Solution:

1. **Understanding Portable Water**:
- Portable water refers to water that is safe for drinking and free from impurities and microorganisms. This definition is correct.
- **Conclusion**: This pair is matched correctly.

2. **Analyzing Biological Oxygen Demand (BOD)**:
- BOD is defined as the amount of oxygen required by aerobic decomposers to break down organic matter in water.
- The statement provided in the question claims that BOD is the amount of oxygen required to oxidize all reducing substrates present in water, which is misleading.
- **Conclusion**: This pair is mismatched.

3. **Examining Oil Spills**:
- Oil spills are indeed the accidental discharge of petroleum from oil tankers and oil refineries.
- **Conclusion**: This pair is matched correctly.

4. **Understanding Eutrophication**:
- Eutrophication refers to the enrichment of water bodies with nutrients, leading to excessive plant growth and a depletion of dissolved oxygen.
- **Conclusion**: This pair is matched correctly.

### Final Conclusion:
Based on the analysis, the mismatched pair is **B** (Biological Oxygen Demand).