During Hatch and stack pathway, PEP combines with `CO_(2)` in the presence of enzyme PEPcase, to form OAA. This process of initial fixation of `CO_(2)` occurs in

Options

A. mesophyll cells
B. bundle sheath cells
C. both a and b
D. None of these

Explanation

### Step-by-Step Solution:

1. **Understanding the Hatch and Slack Pathway**: The Hatch and Slack pathway, also known as the C4 pathway, is a photosynthetic process that occurs in certain plants to efficiently fix carbon dioxide (CO2).

2. **Initial Fixation of CO2**: In this pathway, the initial fixation of CO2 occurs when phosphoenolpyruvate (PEP) combines with CO2. This reaction is catalyzed by the enzyme phosphoenolpyruvate carboxylase (PEPcase).

3. **Formation of OAA**: The product of this reaction is oxaloacetic acid (OAA), which is a four-carbon compound.

4. **Location of the Reaction**: The initial fixation of CO2 by PEP occurs specifically in the **mesophyll cells** of the plant. In contrast, the subsequent reactions of the C4 pathway take place in the bundle sheath cells.

5. **Conclusion**: Therefore, the correct answer to the question is that the initial fixation of CO2 during the Hatch and Slack pathway occurs in the **mesophyll cells**.

### Final Answer:
The process of initial fixation of CO2 occurs in **mesophyll cells**.

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