Given two stationary charged point particles -as in your problem-, the force they feel is expressed by Coulomb's Law, written as:

In which

is the charge of one of the particles,

is the charge of the other particle,

is the distance between the particles, and

is simply a constant.
From Coulomb's Law, lets assume

and

are doubled, what happens to

?

→
So, the force is halved when you double the charge of one particle and the distance.The answer is -2.