A point source of light is used in an experiment on photoelectric effect. Which of the following curves best represents the variation of photoelectric current ‘i’ with distance ‘d’ of the source from the emitter?

The intensity of incident light varies with the distance from the source as follows:

$I \propto \frac{1}{d^2}$, d is the distance from the source and I is the intensity.

so current (i) also varies in a similar way as that of intensity as current is directly proportional to the intensity of the light.

$\mathrm{i}.\mathrm{e}. i \propto \frac{1}{d^2}$.

Hence, the curve d represents the above relation between current and distance.