Final answer:
The Calvin cycle uses ATP and NADPH from the light-dependent reactions, and it sends ADP, Pi, and NADP+ back to be reconverted into ATP and NADPH.
Step-by-step explanation:
The answer to the question "What is sent back to the light reactions from the Calvin cycle?" is d) Both a and b. This is because the Calvin cycle, also known as the light-independent reactions, uses ATP and NADPH generated by the light-dependent reactions as an energy source to synthesize carbohydrates from carbon dioxide. However, the process does not send ATP and NADPH back to the light reactions; rather, it consumes them. Instead, the Calvin cycle generates ADP, Pi, and NADP+, which go back to the light reactions to be re-energized into ATP and NADPH, respectively.