Answer;
C. RuBP
Step-by-step explanation;
The Calvin cycle reactions can be organized into three basic stages: fixation, reduction, and regeneration.
In the stroma, in addition to CO2, two other chemicals are present to initiate the Calvin cycle: an enzyme abbreviated RuBisCO, and the molecule ribulose bisphosphate (RuBP).
Fixation stage; RuBisCO catalyzes a reaction between CO2 and RuBP, which forms a six-carbon compound that is immediately converted into two three-carbon compounds.
Reduction; ATP and NADPH use their stored energy to convert the three-carbon compound, 3-PGA, into another three-carbon compound called G3P.
Regeneration; One of the G3P molecules leaves the Calvin cycle to contribute to the formation of the carbohydrate molecule. The remaining G3P molecules regenerate RuBP, which enables the system to prepare for the carbon-fixation step. ATP is also used in the regeneration of RuBP.