Which of the following occurs in dark reaction of photosynthesis ?

Options

A. Fomation of `ATP`
B. Release of `O_(2)`
C. Release of `H_(2)`
D. Synthesis of `PGA//PGAL`.

Answer

### Step-by-Step Solution:

1. **Understanding Photosynthesis**: Photosynthesis consists of two main stages: the light reactions and the dark reactions (Calvin cycle). Light reactions occur in the thylakoid membranes of chloroplasts and require light energy, while dark reactions occur in the stroma and do not directly require light.

2. **Identifying Light Reactions**: In the light reactions, water (H2O) is split in a process called photolysis, which releases oxygen (O2) and hydrogen ions (H+). This process also generates energy-rich molecules ATP and NADPH.

3. **Understanding Dark Reactions**: The dark reactions, also known as the Calvin cycle, utilize the ATP and NADPH produced in the light reactions along with carbon dioxide (CO2) to synthesize glucose and other sugars. The primary products of the dark reactions include 3-phosphoglycerate (PGA) and glyceraldehyde-3-phosphate (PGAL).

4. **Analyzing the Options**: The question asks which of the following occurs in the dark reaction:
- **Formation of ATP**: This occurs in the light reactions, not in the dark reactions.
- **Release of Oxygen**: This also occurs during the light reactions due to photolysis of water.
- **Release of Hydrogen**: Similar to oxygen, hydrogen is released during the light reactions.
- **Synthesis of PGA or PGAL**: This occurs in the dark reactions (Calvin cycle) as it is the process of converting CO2 into sugars.

5. **Conclusion**: Based on the analysis, the correct answer is the synthesis of PGA or PGAL, which occurs in the dark reactions of photosynthesis.

### Final Answer:
The correct option is the synthesis of PGA or PGAL (Option 4).

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