The correct option is D Reductive carboxylation
The Calvin cycle is a metabolic pathway found in the stroma of the chloroplast in which carbon enters in the form of CO2 and leaves in the form of sugar. The cycle spends ATP as an energy source and consumes NADPH2 as reducing power for adding high energy electrons to make the sugar. There are three phases of the cycle - carbon fixation, carbon reduction and regeneration. In the Calvin cycle, the CO2 undergoes reductive carboxylation, which leads to the synthesis of sugars.