Which of the following shows the particle nature of light?

In a dark room of photography,generally red light is used. The reason is

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?

The curve between current $(i)$ and potential difference $(V)$ for a photo cell will be:

The distance between a light source and a photoelectric cell is $d$. If the distance is decreased to $\frac{d}{2}$,then:

For most metals, threshold frequencies lie in which region?