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.