The correct option is A Carboxylation
Calvin cycle or dark reaction begins with fixation of CO2 into 3 C sugar by the process of carboxylation. RUBISCO, present in the chloroplast stroma catalyzes carboxylation of ribulose-1, 5-biphosphate (RuBP) followed by immediate splitting of unstable product into 3-phosphoglyceric acid. The correct answer is A.