Increase in `CO_(2)` concentration upto .......... Can cause an increase in `CO_(2)` fixation rate, beyond this the level can become damaging over long periods.

Options

A. 0.03 percent
B. 0.04 percent
C. 0.036 percent
D. 0.05 percent

Answer

To answer the question regarding the increase in CO₂ concentration and its effect on the CO₂ fixation rate, we can break down the solution into the following steps:

### Step-by-Step Solution:

1. **Understanding Photosynthesis**:
- Photosynthesis is the process by which plants convert carbon dioxide (CO₂) and water into glucose and oxygen using sunlight. The carbon from CO₂ is essential for forming glucose.

2. **Role of CO₂ in Photosynthesis**:
- CO₂ is a critical limiting factor in the photosynthesis process. Without sufficient CO₂, the rate of photosynthesis will decrease, leading to less glucose production.

3. **Effect of Increasing CO₂ Concentration**:
- As CO₂ concentration increases, the rate of photosynthesis and CO₂ fixation also increases. This is because more CO₂ is available for the plants to use in the photosynthetic process.

4. **Threshold Concentration**:
- However, there is a limit to how much CO₂ concentration can be beneficial. Studies indicate that the threshold concentration for CO₂, beyond which adverse effects may occur, is approximately 0.05%.

5. **Consequences of Excess CO₂**:
- If CO₂ levels exceed this threshold (0.05%), it can lead to detrimental effects on plant cells and negatively impact the photosynthesis process over long periods.

6. **Conclusion**:
- Therefore, the increase in CO₂ concentration up to **0.05%** can enhance the CO₂ fixation rate, but beyond this level, it can become damaging.

### Final Answer:
The increase in CO₂ concentration up to **0.05%** can cause an increase in CO₂ fixation rate; beyond this level, it can become damaging over long periods.

---