Assertion : There is not time lag between the incidence of light and emission of photoelectron.
Reason : By increasing intensity of incident radiation , the number of emitted electrons increases.

Options

A. If both assertion and reason are correct and reason is a correct explanation of the assertion.
B. If both assertion and reason are correct but reason is not the correct explanation of assertion.
C. If assertion is correct but reason is incorrect
D. If assertion is incorrect but reason is correct.

Answer

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that there is no time lag between the incidence of light and the emission of photoelectrons. This means that when light strikes a photoelectric material, the ejection of electrons occurs almost instantaneously.
- This is consistent with the photoelectric effect, where photons (light particles) interact with electrons in a material, leading to immediate emission if the photon energy exceeds the work function of the material.

2. **Understanding the Reason**:
- The reason states that by increasing the intensity of the incident radiation, the number of emitted electrons increases.
- Intensity is related to the number of photons striking the surface per unit time. More photons mean more chances for electrons to absorb energy and be emitted, thus increasing the number of emitted electrons.

3. **Evaluating the Relationship**:
- While both the assertion and reason are correct, they address different aspects of the photoelectric effect.
- The assertion focuses on the instantaneous nature of electron emission, while the reason discusses the effect of intensity on the number of emitted electrons.
- There is no direct causal relationship between the two statements. The lack of time lag does not depend on the intensity of the incident light.

4. **Conclusion**:
- Both the assertion and reason are correct, but the reason does not explain the assertion. Therefore, the correct answer is that both the assertion and reason are true, but the reason is not the correct explanation of the assertion.

### Final Answer:
Both assertion and reason are correct, but the reason is not the correct explanation of the assertion.