During the Calvin cycle, rubisco's function is to:

a) fix carbon to RuBP (ribulose bisphosphate).
b) reduce NADPH.
c) form RuBP (ribulose bisphosphate) while releasing ATP.
d) regenerate RuBP (ribulose bisphosphate) molecules for carbon fixation.
e) catalyze the addition of glyceraldehyde 3-phosphate to acetyl-CoA.

RuBisCO's function during the Calvin cycle is to catalyze the fixation of carbon dioxide to ribulose bisphosphate (RuBP), which is the first major step of carbon fixation.

Explanation:

During the Calvin cycle, the enzyme ribulose bisphosphate carboxylase, commonly known as RuBisCO, performs the critical role of catalyzing the first major step of carbon fixation.

RuBisCO facilitates the addition of carbon dioxide (CO₂) to ribulose bisphosphate (RuBP), resulting in the production of 3-phosphoglycerate (3-PGA). This stage is known as the fixation or carboxylation stage. It is essential to the synthesis of organic molecules in the plant. Therefore, the correct answer to the student's question is: a) fix carbon to RuBP (ribulose bisphosphate).