(N/A) The photoelectric equation is given by $h\nu = \phi_0 + eV_0$,where $h$ is Planck's constant,$\nu$ is the frequency of incident radiation,$\phi_0$ is the work function,and $V_0$ is the stopping potential.
Rearranging for $V_0$,we get $V_0 = (\frac{h}{e})\nu - \frac{\phi_0}{e}$.
Comparing this with the equation of a straight line $y = mx + c$,the slope $m$ is $\frac{h}{e}$.
The slope of the $V_0 - \nu$ graph is $\frac{h}{e}$,which is a universal constant.
Since $h$ (Planck's constant) and $e$ (charge of an electron) are universal constants,the slope does not depend on the type of material used for the photosensitive surface.