Question

what molecule combines with the 5 - carbon rubp to form a 6 - carbon compound during the calvin cycle?

Explanation:

In the Calvin - cycle, carbon dioxide ($CO_2$) combines with the 5 - carbon ribulose - 1,5 - bisphosphate (RuBP) to form a 6 - carbon compound which is highly unstable and quickly splits.

Answer:

Carbon Dioxide