Answer: a. CO2
Explanation: CO2 fixation is the first step in the Calvin cycle. Calvin cycle is driven by the products of the light reactions of photosynthetic carbohydrate synthesis. CO2 is a product of the light reactions. In the fixation of CO2, three molecules of CO2 is incorporated into three molecules of ribulose 1, 5-bisphosphate (5 carbon sugar) to form six molecules of 3- phosphoglycerate (3 carbon molecule). This reaction is catalyzed by an enzyme known as ribulose 1, 5- bisphosphate caboxylase (rubisco).
