Answer
(c) it decreses
Work Step by Step
Stopping voltage is a voltage V corresponding to the maximum kinetic energy .$ K_{max}$ of the electron eV.
Since in photoelectric effect maximum kinetic energy depends upon the frequency of incident light
.$ K_{max} =hf - \phi_{0}$
where f is frequency of incident light and $\phi_{0}$ is workfunction of metal surface.
so stopping voltage decreases when the light frequency is decreased.
