PEP carboxylase, or phosphoenolpyruvate carboxylase, is an enzyme that plays a crucial role in carbon dioxide (CO2) fixation in plants. It is involved in the initial step of the C4 photosynthetic pathway. Let's delve into the details of the given answer option 'C' explaining why it is correct.

PEP Carboxylase Fixes CO2 More Efficiently at Lower Atmospheric CO2 Concentrations:
- PEP carboxylase has a higher affinity for CO2 than RuBP carboxylase, also known as Rubisco.
- This means that PEP carboxylase can effectively fix CO2 even at lower atmospheric concentrations, making it more efficient under conditions of limited CO2 availability.
- In contrast, Rubisco, which is the primary enzyme responsible for CO2 fixation in C3 plants, has a lower affinity for CO2 and requires higher atmospheric concentrations to function optimally.
- This difference in CO2 affinity allows PEP carboxylase to effectively perform CO2 fixation in C4 plants, which have adapted to low CO2 conditions, such as hot and dry environments.

C4 Plants and the Role of PEP Carboxylase:
- C4 plants have evolved a specialized mechanism to enhance CO2 fixation and reduce photorespiration, a wasteful process that occurs in C3 plants under high temperatures and low CO2 conditions.
- In C4 plants, CO2 is initially fixed into a four-carbon compound called oxaloacetate by PEP carboxylase in the mesophyll cells.
- This four-carbon compound is then transported to bundle sheath cells, where it is decarboxylated to release CO2, which is then used by Rubisco to perform CO2 fixation through the Calvin cycle.
- The spatial separation of PEP carboxylase in the mesophyll cells and Rubisco in the bundle sheath cells allows for efficient CO2 fixation and minimizes photorespiration.
- PEP carboxylase catalyzes the initial step of CO2 fixation in C4 plants, making it a crucial enzyme in this pathway.

Conclusion:
PEP carboxylase is involved in CO2 fixation in both C3 and C4 plants, but it plays a more prominent role in C4 plants. It has a higher affinity for CO2 than Rubisco, allowing it to fix CO2 more efficiently at lower atmospheric concentrations. This characteristic is particularly advantageous for C4 plants, which have adapted to low CO2 conditions. Therefore, option 'C' stating that PEP carboxylase is capable of fixing CO2 more efficiently at lower atmospheric CO2 concentrations than Rubisco is the correct answer.