The correct option is A RuBisCO
RuBisCO or Ribulose Bisphosphate Carboxylase/Oxygenase is the most abundant enzyme in the world. It is responsible for the first step of the dark reactions in the synthesis of carbohydrates, which is the carbon fixation step. In this step, carbon dioxide from the atmosphere is fixed on to ribulose bisphosphate, thus starting the Calvin cycle. The end-product of the dark reaction is carbohydrates.