The correct option is A The primary fixation of CO2 is mediated via PEP carboxylase
In C4 plants, initial fixation of carbon dioxide occurs in mesophyll cells. The primary acceptor of CO2 is phosphoenol pyruvate or PEP. It combines with carbon dioxide in the presence of PEP carboxylase or PEPcase to form oxaloacetate.
C4 plants are more efficient in picking up CO2 even when it is found in low concentration because of the high affinity of PEPcase.