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

Options

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

Answer

To determine which pair is not correctly matched regarding mineral nutrition in plants, we will analyze each option provided.

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks us to identify the incorrect pairing of essential nutrients and their functions in plants.

2. **Analyzing Option A**:
- **Nutrient**: Molybdenum
- **Function**: Required for nitrogen fixation.
- **Evaluation**: This is correct. Molybdenum is indeed a micronutrient essential for nitrogen fixation in plants.

3. **Analyzing Option B**:
- **Nutrient**: Manganese
- **Function**: Required for the integrity of chloroplast membranes and for oxygen release in photosynthesis.
- **Evaluation**: This is also correct. Manganese plays a crucial role in the photolysis of water during photosynthesis, which is essential for oxygen release.

4. **Analyzing Option C**:
- **Nutrient**: Iron
- **Function**: Required for the development of chlorophyll and its pigment.
- **Evaluation**: This statement is correct as well. Iron is vital for the synthesis of chlorophyll, which is necessary for photosynthesis.

5. **Analyzing Option D**:
- **Nutrient**: Magnesium
- **Function**: Required for osmosis and ionic balance and in the opening and closing of stomata.
- **Evaluation**: This is incorrect. While magnesium does play a role in osmosis and ionic balance, its primary function is to act as a central atom in the chlorophyll molecule, which is crucial for photosynthesis. The functions mentioned (osmosis and stomatal movement) are primarily associated with potassium ions.

6. **Conclusion**: The incorrect pairing is found in Option D, where the function attributed to magnesium is not accurate.

### Final Answer:
The pair that is not correctly matched is **Option D** (Magnesium required for osmosis and ionic balance and in opening and closing of stomata).