Photosynthesis in C4 plants is relatively less limited by atmospheric CO2 levels because :

C4 plants have evolved a mechanism to reduce photorespiration and increase the efficiency of CO2 fixation. This is achieved through the use of PEP (phosphoenolpyruvate) carboxylase, an enzyme that has a higher affinity for CO2 than the enzyme RUBISCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) used in C3 plants. PEP carboxylase fixes CO2 into a 4-carbon molecule (oxaloacetate or malate), which is then transported into the bundle sheath cells where the CO2 is released and fixed by RUBISCO. This process effectively concentrates CO2 in the bundle sheath cells, reducing the limitations imposed by low atmospheric CO2 levels. Therefore, the correct answer is option (B) the primary fixation of CO2 is mediated via PEP carboxylase. Think of it this way: C4 plants use PEP carboxylase to initially fix CO2 into a 4-carbon compound, which helps to concentrate CO2 in the bundle sheath cells, thus reducing the impact of low CO2 levels.