Introduction to C4 Pathway
The C4 pathway is a crucial photosynthetic process that enhances CO2 fixation, especially in hot and arid environments. Understanding where this fixation occurs is essential for grasping plant physiology.

PEPCase Function
- Phosphoenolpyruvate Carboxylase (PEPCase) is the enzyme responsible for the initial fixation of CO2 in the C4 pathway.
- It catalyzes the reaction between CO2 and phosphoenolpyruvate (PEP) to form oxaloacetate, a four-carbon compound.

Location of CO2 Fixation
- CO2 fixation by PEPCase occurs primarily in the **mesophyll cells** of the leaves.
- The mesophyll cells are strategically located to capture atmospheric CO2, making them the first point of contact in the C4 photosynthetic pathway.

Role of Mesophyll Cells
- Mesophyll cells contain high concentrations of PEPCase, facilitating efficient CO2 uptake.
- After CO2 fixation, the oxaloacetate produced is converted into malate or aspartate, which is then transported to bundle sheath cells.

Bundle Sheath and Final Steps
- While the final steps of the C4 pathway occur in **bundle sheath cells**, where the Calvin cycle takes place, CO2 fixation itself is initiated in the mesophyll.
- This spatial separation is key to minimizing photorespiration and optimizing photosynthetic efficiency.

Conclusion
In summary, the fixation of CO2 by PEPCase in the C4 pathway occurs in the **mesophyll cells**, making option 'B' the correct answer. This adaptation allows plants to thrive in conditions that would otherwise limit photosynthesis.