Carbon dioxide is the major limiting factor for photosynthesis. With respect to this . choose the correct Statement from the following.

Options

A. Crops like tomato and bell pepper use increased levels of `CO_2` for higher productivity and are hence called greenhouse crops.
B. The current availability of `CO_2` levels is limiting to `C_4` plants .
C. `C_4` plants show saturation at about `450ppm` while `C_3` responds to increased `CO_2` concentration and saturation is seen only beyond `360 ppm`.
D. Increase in concentration up to 0.03 per cent can cause an increases in `CO_2` fixation rates.

Answer

To solve the question regarding carbon dioxide as a major limiting factor for photosynthesis, we will analyze each statement provided in the options and determine which one is correct.

### Step-by-Step Solution:

1. **Understanding Carbon Dioxide's Role in Photosynthesis**:
- Carbon dioxide (CO2) is essential for photosynthesis, and its concentration in the atmosphere is relatively low (approximately 0.04%).
- Increasing CO2 concentration can enhance the rate of photosynthesis up to a certain point (around 0.05%).

**Hint**: Remember that photosynthesis requires CO2, and its concentration can influence the rate of this process.

2. **Analyzing the First Option**:
- The statement claims that crops like tomato and bell pepper utilize increased levels of CO2 for higher productivity and are termed greenhouse crops.
- This is true because these crops thrive in environments with higher CO2 concentrations, leading to increased photosynthesis and productivity.

**Hint**: Consider how certain crops respond positively to elevated CO2 levels, especially in controlled environments.

3. **Analyzing the Second Option**:
- This option states that the current availability of CO2 levels is limiting to C4 plants.
- This is incorrect. C4 plants, such as maize and sugarcane, are adapted to efficiently utilize low levels of CO2 due to the presence of the enzyme PEP carboxylase.

**Hint**: Recall the differences between C3 and C4 plants regarding their adaptation to CO2 levels.

4. **Analyzing the Third Option**:
- The statement suggests that C4 plants show saturation at about 450 ppm while C3 plants respond to increased CO2 concentration with saturation seen only beyond 360 ppm.
- This is incorrect as the saturation point for C4 plants is actually higher than that of C3 plants.

**Hint**: Be aware of the saturation points for CO2 in different plant types.

5. **Analyzing the Fourth Option**:
- This option claims that an increase in CO2 concentration up to 0.03% can cause an increase in CO2 fixation rate.
- This is misleading because significant increases in CO2 fixation rates typically occur when concentrations reach around 0.05%.

**Hint**: Understand the specific concentration thresholds that affect photosynthesis rates.

### Conclusion:
After evaluating all the options, the correct statement is:

**Correct Answer**: Option 1 - "Crops like tomato and bell pepper use increased level of carbon dioxide for higher productivity and are hence called greenhouse crops."