Answer:
The magnitude of the electrostatic force decreases by a factor 9
Step-by-step explanation:
The electrostatic force between two charges is given by:

where
k is the Coulomb's constant
q1 and q2 are the two charges
d is the distance between the two charges
We see that the magnitude of the force is inversely proportional to the square of the distance. If the distance is increased to 3d: d' = 3d, the new electrostatic force would be:

So, the electrostatic force decreases by a factor 9.