The ratio of CO₂, NADPH₂, and ATP molecules required to synthesize starch from the output of the Calvin cycle is:

Explanation

To synthesize one molecule of glucose through the Calvin cycle, 6 molecules of CO₂, 12 molecules of NADPH₂, and 18 molecules of ATP are needed. Glucose is the monomeric unit of starch, and thus the base ratio of 6:12:18 reflects the necessary inputs for glucose formation. Starch is a polymer of glucose, indicating that the Calvin cycle's foundational ratio is essential for forming the building blocks of starch. Option C correctly identifies this ratio. The other options either underestimate or overestimate the required amounts of each molecule, reflecting a misunderstanding of the biochemical requirements for glucose and subsequent starch synthesis.