With reference to mineral nutrition in plants, which one of the following pair is not correctly matched?

Options

A. Molybdenum - Required for nitrogen fixation and nitrate reduction
B. Manganese - Required for intergrity of chloroplast membrane and for oxygen release in photosynthesis
C. Iron - Required for development of chlorophyll and its pigments.
D. Magnesium - involved in osmosis and ionic balance, and in opening and closing of stomata.

Answer

To determine which pair related to mineral nutrition in plants is not correctly matched, we will analyze each option provided in the question step by step.

### Step-by-Step Solution:

1. **Option A: Molybdenum**
- Molybdenum is required for nitrogen fixation and nitrate reduction.
- It is a component of the enzyme nitrogenase, which is essential for nitrogen fixation, and it also plays a role in the enzyme nitrate reductase, which is involved in the reduction of nitrates.
- **Conclusion**: This option is correctly matched.

2. **Option B: Manganese**
- Manganese is required for the integrity of the chloroplast membrane and for oxygen release during photosynthesis.
- It helps maintain the structural integrity of chloroplasts and is involved in the water-splitting reaction that releases oxygen in photosynthesis.
- **Conclusion**: This option is correctly matched.

3. **Option C: Iron**
- Iron is required for the development of chlorophyll and its pigments.
- It is crucial for the synthesis of chlorophyll, and its deficiency leads to chlorosis, which is characterized by yellowing of leaves.
- **Conclusion**: This option is correctly matched.

4. **Option D: Magnesium**
- Magnesium is said to be involved in osmosis and ionic balance and in the opening and closing of stomata.
- However, potassium is primarily responsible for osmosis and ionic balance in plants, while magnesium is more involved in the formation of chlorophyll and enzyme activation.
- **Conclusion**: This option is incorrectly matched.

### Final Answer:
The pair that is not correctly matched is **Option D: Magnesium**.

---