The correct option is A Mg
Both RUBISCO and PEP carboxylase are important enzymes of dark reaction of photosynthesis. RUBISCO catalyzes carboxylation in C3 plants. In the presence of magnesium activated RUBISCO, carbon dioxide combines with ribulose bis phosphate to form phosphoglyceric acid. In C4 plants, mesophyll cells fix carbon dioxide with the help of phospho enol pyruvate in the presence of magnesium activated phospho-enol pyruvate carboxylase.