Given:
Distance between particles = 1 m
Force exerted = 1 N
Let's find the force on each particle if he magnitude of each charge is doubled.
Apply the Columb's Law:

Where:
F is the force
Q1 is the charge of particle 1
Q2 is the charge of particle 2
r is the distance between the particles.
Rewrite the equation for Q1Q2

If the magnitude is doubled, we have:
Q1 = 1 x 2 = 2Q1
Q2 = 1 x 2 = 2Q2
Hence, we have:

Rewrite the equation for Q1Q2:

We have the equations for Q1Q2:

Eliminate the equal sides of both eqautions and equate:

Make F subject of the equation:
Cross multiply

Therefore, when the magnitude of each charge is doubled, the force on each particle will be 4 N
ANSWER:
4 N