A point source causes photoelectric effect from a small metal plate. Which of the following curves may represent the saturation photocurrent as a function of the distance between the source and the metal?

From the given curves ,curve (d) is correct.

As the relation between intensity (I) of light and distance (r) is
$I\propto \frac{1}{r^2}$
As the distance between the source and the metal is increased, it will result in decrease in the intensity of light. As the saturation current is directly proportional to the intensity of light ($i\propto I$), it can be concluded that current varies as $i\propto \frac{1}{r^2}$. Thus, curve d is correct.