Final answer:
The enzyme responsible for the first stage of the Calvin cycle is called RuBisCO.
Step-by-step explanation:
The enzyme responsible for the first stage of the Calvin cycle is called RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase).
In the stroma, RuBisCO incorporates carbon dioxide (CO₂) into an organic molecule called 3-PGA. This is the process of carbon fixation, which is the first step in the Calvin cycle.