1. Understand the Assertion (A)

In the photoelectric effect, emitted photoelectrons have a range of kinetic energies up to a maximum value: $K_{\max}=h\nu-\phi$ where:

• $h\nu$ = energy of incident photon
• $\phi$ = work function of the metal

If we apply a sufficiently negative potential to the photoemissive surface (relative to the collector), the emitted electrons are repelled and may fail to reach the collector. This retarding potential can suppress the photocurrent completely when it reaches the stopping potential.

So, Assertion (A) is true.

2. Understand the Reason (R)

The stopping potential $V_s$ is related to the maximum kinetic energy by $eV_s=K_{\max}=h\nu-\phi$ Thus, $V_s=\frac{h}{e}\nu-\frac{\phi}{e}$

This is a linear relation between stopping potential and frequency $\nu$.

Hence, Reason (R) is also true.

3. Check whether (R) correctly explains (A)

• Assertion (A) says that emission/effective collection of photoelectrons can be suppressed by applying a sufficiently negative potential.
• Reason (R) states that the stopping potential varies linearly with frequency.

The actual explanation of (A) is that a negative potential opposes the motion of emitted electrons and, if large enough, prevents even the most energetic electrons from reaching the collector.

The linear dependence of stopping potential on frequency is a correct statement, but it does not explain why a negative potential suppresses the photoelectric effect/current.

So, both are true, but (R) is not the correct explanation of (A).

4. Correct Option

Therefore, the correct answer is: $\boxed{\text{B}}$