Which of the following steps involves reduction taking place in the absence of light?

Options

A. Formation of NADPH2
B. Addition of carbon to RUBP
C. Formation of glucose from PGA
D. Regeneration of RUBP

Explanation

To determine which step involves reduction taking place in the absence of light, we can analyze the options provided:

1. **Formation of NADPH2**: This step occurs during the light reactions of photosynthesis, where light energy is used to convert NADP+ into NADPH2. Since this process requires light, it does not occur in the absence of light. Therefore, this option is not correct.

2. **Addition of carbon to RUBP**: This step is part of the Calvin cycle, which also occurs during the light reactions. The addition of carbon dioxide to ribulose bisphosphate (RUBP) is facilitated by the enzyme RuBisCO and requires light energy. Thus, this option is also not correct.

3. **Formation of glucose from PGA**: This step occurs during the dark reactions (Calvin cycle) of photosynthesis. In this process, phosphoglyceric acid (PGA) is converted into glucose (C6H12O6) using NADPH and ATP produced during the light reactions. This process does not require light and involves the reduction of PGA to form glucose. Therefore, this option is correct.

4. **Regeneration of RUBP**: This step is also part of the Calvin cycle and occurs during the light reactions. It involves the regeneration of RUBP from the products of the dark reactions, which requires energy derived from light. Hence, this option is not correct.

Based on the analysis, the step that involves reduction taking place in the absence of light is the **formation of glucose from PGA**.

### Summary of Steps:
1. Analyze the first option (Formation of NADPH2) - Not correct, as it requires light.
2. Analyze the second option (Addition of carbon to RUBP) - Not correct, as it occurs in light reactions.
3. Analyze the third option (Formation of glucose from PGA) - Correct, as it occurs in dark reactions and involves reduction.
4. Analyze the fourth option (Regeneration of RUBP) - Not correct, as it requires light.