Which of the following phenomena can explain the quantum nature of light?

From the photoelectric effect experiment,select the correct statement.

The graph shows the variation of photocurrent with anode potential for four different radiations. Let $I_a, I_b, I_c$ and $I_d$ be the intensities and $f_a, f_b, f_c$ and $f_d$ be the frequencies for the curves $a, b, c$ and $d$ respectively,then

In the photoelectric effect,a photocell is used for the conversion of ..........

If the intensity of incident radiation in a photocell is increased, the stopping potential

The curves $(a), (b), (c)$ and $(d)$ show the variation between the applied potential difference $(V)$ and the photoelectric current $(i)$,at two different intensities of light $(I_1 > I_2)$. In which figure is the correct variation shown?