The wave picture of light has failed to explain which of the following phenomena?

When a monochromatic point source of light is at a distance of $0.2\, m$ from a photoelectric cell,the cut-off voltage and the saturation current are respectively $0.6\, V$ and $18.0\, mA$. If the same source is placed $0.6\, m$ away from the photoelectric cell,then

Light of frequency $1.5 \nu_0$ (where $\nu_0$ is the threshold frequency) is incident on a photosensitive material. If the frequency is halved and the intensity is doubled,what happens to the photocurrent?

The quantum nature of light is explained by which of the following phenomena?

The cathode of a photocell is changed such that the work function changes from $w_1$ to $w_2$ $(w_2 > w_1)$. If the saturation currents before and after the change are $I_1$ and $I_2$ respectively,and all other conditions (such as incident light intensity and frequency) remain unchanged,then (assuming $h\nu > w_2$):

The number of photoelectrons emitted for light of a frequency $\nu$ (higher than the threshold frequency $\nu_0$) is proportional to