The correct option is A Mesophyll cells
Surrounding the bundle sheath cells are mesophyll cells in which a much more active enzyme, phosphoenolpyruvate (PEP) carboxylase, fixes CO2 (but as HCO3) into oxaloacetate, a C4 acid. The C4acid diffuses to the bundle sheath cell, where it is decarboxylated and refixed in the normal C3 pathway. So the correct option is 'Mesophyll cells'.