A point source induces the photoelectric effect on a small metal...

The saturation photocurrent $i$ is directly proportional to the intensity $I$ of the incident light on the metal plate.

For a point source of light, the intensity $I$ at a distance $r$ is given by $I = \dfrac{P}{4\pi r^2}$, where $P$ is the power of the source. Thus, the intensity is inversely proportional to the square of the distance, i.e., $I \propto \dfrac{1}{r^2}$.

Therefore, the saturation photocurrent $i$ is also inversely proportional to the square of the distance:

$i \propto \dfrac{1}{r^2}$

The graph of $i$ versus $r$ will be a curve that decreases with distance and is concave upwards (since $\dfrac{d^2i}{dr^2} > 0$).

Among the given curves, curve d correctly represents this relationship.