The calvin cycle of photosynthesis begins when a. light becomes available. b. electrons leave a photosystem. c. the reduction of carbon dioxide occurs. d. carbon dioxide is attached to rubp.

The Calvin Cycle of photosynthesis begins when carbon dioxide reacts with RuBP, which is a 5-carbon carbohydrate. The enzyme called enzyme called Rubisco joins the carbon dioxide and RuBP. So the answer to your question would be D. carbon dioxide is attached to RuBP