The correct answer is:
sixteen times
In fact, the distance between charge q and the source S is 1 unit. Instead, the distance between charge p and the source S is 4 units. The magnitude of the electrostatic force is inversely proportional to the square of the distance between the charge and the source:
where r is the distance. If we take the force between q and S as reference, we have r=1, so that
while the force between p and S is
Therefore, we see that the force exerted between q and S is 16 times the force exerted between p and S.