Answer is A. Calvin cycle.
Calvin cycle reactions or light independent process of photosynthesis occurs in the stroma (Ground substance) of the chloroplast. In the stroma enzymes are present for the Calvin cycle reactions. Carbon dioxide (CO₂) enters into the leaves through stomata and diffuses into stroma of the chloroplast. In the stroma carbon atoms from CO₂ is fixed into sugar (carbohydrate) with the help of energy from ATP and NADPH.
