Chapter 15 - Electric Charge, Forces, and Fields - Learning Path Questions and Exercises - Multiple Choice Questions - Page 554: 7

(a) decreases

Magnitude of force between 2 point charges by Coulomb's law is :- $F=k\frac{q_{1}q_{2}}{r^{2}}$ notations having usual meanings. When the distance, increases (r), the force decreases. So, (a) is the answer.