In C3 plants, what is the role of ribulose-1,5-bisphosphate (RuBP) during carbon fixation?

• A. RuBP is a product of the Calvin cycle that stores energy.
• B. RuBP is a pigment that absorbs light energy for photosynthesis.
• C. RuBP is an enzyme that catalyzes the fixation of carbon dioxide.
• D. RuBP acts as a substrate that combines with CO2 to form phosphoglycerate.

Answer: (D)