Carbon dioxide joins the photosynthetic pathway in

Options

A. PS-I
B. PS-II
C. light reaction
D. dark reaction

Explanation

**Step-by-Step Solution:**

1. **Understanding Photosynthesis**: Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy in the form of glucose. It primarily occurs in two stages: the light reactions and the dark reactions (Calvin cycle).

2. **Identifying the Stages**:
- **Light Reactions**: These occur in the thylakoid membranes of the chloroplasts and require light. During this phase, light energy is converted into chemical energy in the form of ATP and NADPH. Carbon dioxide (CO2) does not directly participate in this stage.
- **Dark Reactions (Calvin Cycle)**: These occur in the stroma of the chloroplasts and do not require light directly. Instead, they utilize the ATP and NADPH produced in the light reactions to convert CO2 into glucose and other carbohydrates.

3. **Analyzing the Options**:
- **PS1 and PS2**: These refer to Photosystem I and Photosystem II, which are part of the light reactions. They are involved in capturing light energy and do not involve CO2.
- **Light Reaction**: As previously mentioned, this stage does not involve CO2 directly.
- **Dark Reaction**: This is where CO2 is fixed and converted into carbohydrates using the energy from ATP and NADPH.

4. **Conclusion**: Based on the understanding of the photosynthetic pathway, carbon dioxide joins the photosynthetic pathway during the dark reactions, where it is utilized to synthesize carbohydrates.

5. **Final Answer**: The correct option is D, Dark Reaction.

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