Assertion (A) is true because a sufficiently negative potential can repel and stop the emitted electrons from leaving the surface. Reason (R) is also true because the stopping potential Vstop varies linearly with the frequency ν of the incident light (Vstop =ehν−eϕ). However, (R) does not directly explain why a negative potential suppresses electron emission; it only shows how much potential is needed for different frequencies. Therefore, both (A) and (R) are true, but (R) is not the correct explanation of (A).