Assertion (A) : Kinetic energy of photoelectrons emitted by a photosensitive surface depends upon the frequency of incident photon.
Reason (R ): The ejection of electrons from metallic surface is possible with frequency of incident photon below the threshold frequency.

Options

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

Explanation

To solve the assertion and reason question, we need to analyze both the assertion (A) and the reason (R) provided.

### Step 1: Analyze the Assertion (A)
The assertion states: "Kinetic energy of photoelectrons emitted by a photosensitive surface depends upon the frequency of incident photon."

- According to the photoelectric effect, the kinetic energy (KE) of the emitted photoelectrons can be expressed by the equation:
\[ KE = h\nu - \phi \]
where:
- \( KE \) is the kinetic energy of the emitted electrons,
- \( h \) is Planck's constant,
- \( \nu \) is the frequency of the incident photon,
- \( \phi \) is the work function of the material (the minimum energy required to remove an electron from the surface).

- From this equation, we can see that the kinetic energy of the photoelectrons is directly related to the frequency of the incident photons. Therefore, if the frequency increases, the kinetic energy of the emitted electrons also increases, provided that the frequency is above the threshold frequency.

**Conclusion for Assertion (A): True**

### Step 2: Analyze the Reason (R)
The reason states: "The ejection of electrons from a metallic surface is possible with frequency of incident photon below the threshold frequency."

- The threshold frequency (\( \nu_0 \)) is defined as the minimum frequency required to eject electrons from the surface of a material. If the frequency of the incident photon is below this threshold frequency, the energy of the photon is insufficient to overcome the work function (\( \phi \)) of the material.

- Therefore, if the frequency is below the threshold frequency, no electrons will be emitted, and thus, the statement in the reason is incorrect.

**Conclusion for Reason (R): False**

### Final Conclusion
- Assertion (A) is true.
- Reason (R) is false.

Thus, the correct answer is that the assertion is true, and the reason is false.