`C_(4)` plant shows efficiency even in

Options

A. Low concentration of `CO_(2)`
B. Low temperature
C. High `CO_(2)` concentration
D. At low water availability.

Answer

### Step-by-Step Solution:

1. **Understanding C4 Plants**: C4 plants are a type of plant that have a unique mechanism for photosynthesis, allowing them to efficiently capture carbon dioxide (CO2) and convert it into energy. This mechanism is particularly advantageous in certain environmental conditions.

2. **Key Characteristics of C4 Plants**:
- C4 plants do not undergo photorespiration, which is a wasteful process that occurs in some plants when CO2 levels are low.
- They have a specialized pathway that concentrates CO2 around the enzyme Rubisco, which is crucial for the photosynthesis process.

3. **Analyzing the Options**:
- **Option A: Low concentration of CO2**: C4 plants can efficiently perform photosynthesis even when CO2 levels are low due to their unique mechanism of concentrating CO2 around Rubisco.
- **Option B: Low temperature**: C4 plants are generally more efficient in warmer temperatures, so this option is not correct.
- **Option C: High CO2 concentration**: While C4 plants can utilize high CO2 concentrations, their efficiency is not specifically tied to this condition.
- **Option D: Low water availability**: C4 plants are more drought-resistant than C3 plants, but they do not show efficiency specifically due to low water availability.

4. **Conclusion**: The correct answer is **Option A: Low concentration of CO2**. C4 plants are efficient in photosynthesis even when CO2 levels are low.