Assertion (A): The rate of photosynthesis increases with increase in light intensity only up to a point.
Reason ( R): Photosynthesis includes dark phase

Options

A. Both A and R are true and R is the correct explanation for A.
B. Both A and R are true but R is not the correct explanation for A.
C. A is true and R is false.
D. A is false and R is true.

Explanation

### Step-by-Step Solution

1. **Understanding the Assertion (A)**:
- The assertion states that the rate of photosynthesis increases with an increase in light intensity only up to a certain point. This means that while light is a crucial factor for photosynthesis, there is a limit beyond which further increases in light intensity do not lead to an increase in the rate of photosynthesis.

2. **Understanding the Reason (R)**:
- The reason provided states that photosynthesis includes a dark phase. This refers to the Calvin cycle, where the products of the light-dependent reactions (ATP and NADPH) are used to synthesize glucose from carbon dioxide.

3. **Connecting Assertion and Reason**:
- The assertion is true because, initially, as light intensity increases, the rate of photosynthesis also increases due to more energy being available for the light-dependent reactions. However, once the light intensity reaches a certain threshold, other factors (like CO2 concentration or temperature) may become limiting, causing the rate of photosynthesis to plateau.
- The reason is also true because the dark phase (Calvin cycle) is essential for the overall process of photosynthesis, as it utilizes the products generated during the light phase.

4. **Conclusion**:
- Both the assertion and the reason are correct, and the reason explains the assertion. Therefore, the correct answer is that both A and R are true, and R is the correct explanation of A.