Internal factor affecting photosynthesis is / are :

Options

A. Number and size of leaf
B. Age of leaf and orientation
C. Internal `CO_2` concentration and amount of chlorophyll
D. All the above

Answer

To determine the internal factors affecting photosynthesis, let's analyze each option provided in the question step by step.

### Step-by-Step Solution:

1. **Understanding Photosynthesis**:
Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy, primarily in the form of glucose. This process involves various internal factors that can influence its efficiency.

2. **Evaluating the Options**:
- **Option 1: Number and Size of Leaves**:
- The number and size of leaves are crucial because more leaf area means more surface for light absorption. Larger leaves can capture more sunlight, which can enhance the rate of photosynthesis.
- **Conclusion**: This is an internal factor affecting photosynthesis.

- **Option 2: Leaf Age and Orientation**:
- The age of the leaf affects its chlorophyll content. Younger leaves typically have higher chlorophyll concentrations, which are essential for capturing light energy. Additionally, the orientation of leaves can influence how much sunlight they receive.
- **Conclusion**: This is also an internal factor affecting photosynthesis.

- **Option 3: Chlorophyll Content**:
- Chlorophyll is the pigment responsible for absorbing light. A higher chlorophyll content directly correlates with an increased rate of photosynthesis.
- **Conclusion**: This is an internal factor affecting photosynthesis.

- **Option 4: Internal Carbon Dioxide Concentration**:
- The concentration of carbon dioxide inside the leaf is crucial for photosynthesis, as it is one of the raw materials used in the process. Higher concentrations can enhance the rate of photosynthesis.
- **Conclusion**: This is an internal factor affecting photosynthesis.

3. **Final Evaluation**:
- Since all the options discussed above are internal factors that affect photosynthesis, the correct answer is **"All of the above"**.

### Answer:
The correct answer is **Option 4: All of the above**.