Which of the following is not an external factor influencing photosynthesis?

Options

A. `CO_(2)` concentration
B. `O_(2)` concentration
C. Availability of water
D. Chlorophyll concentration

Explanation

To determine which of the following is not an external factor influencing photosynthesis, we need to analyze each option provided:

1. **CO2 concentration**: Carbon dioxide is a gas that plants take in from the atmosphere during photosynthesis. It is an external factor because it comes from outside the plant.

2. **O2 concentration**: Oxygen is a byproduct of photosynthesis, and while plants do release oxygen, the concentration of oxygen in the environment can influence the rate of photosynthesis. Thus, it is also considered an external factor.

3. **Availability of water**: Water is essential for photosynthesis and is absorbed by the plant from the soil. It is another external factor since it comes from the environment.

4. **Chlorophyll concentration**: Chlorophyll is the green pigment found inside the chloroplasts of plant cells. It is crucial for capturing light energy during photosynthesis, but it is an internal factor because it is located within the plant's cells.

Based on this analysis, the factor that is not external is:

**Answer**: Chlorophyll concentration (Option D).

### Step-by-Step Solution:
1. **Identify the factors influencing photosynthesis**: Understand that photosynthesis is influenced by various environmental conditions.
2. **Analyze each option**: Evaluate whether each option is an external or internal factor.
- CO2 concentration: External
- O2 concentration: External
- Availability of water: External
- Chlorophyll concentration: Internal
3. **Determine the correct answer**: Since chlorophyll concentration is internal, it is the factor that does not influence photosynthesis externally.