Correct Answer  Option 3 : 18 N
CONCEPT:

Coulomb’s law is an inverse square law of force that describes the electrostatic force between charged particles.

Electrostatic Force: between two charges q1 and q2 at the distance of R is given by:
\(F=\frac{1}{4\pi ϵ_0}\frac{q_1q_2}{R^2} \)
where F is the electrostatic force, q1 and q2 are the charges, and ϵ0 is the electrical permittivity of the vacuum.
CALCULATION:
Given that q_{1} = 5μC and q_{2} = 4μC R = 10cm = 0.1m;
We know \(F=\frac{1}{4\pi ϵ_0}\frac{q_1q_2}{R^2} \)
\(F=9×10^9×\frac{5μC \times 4μC}{0.1^2} \)
\(F=9×10^9×\frac{5 \times 10^{6} \times 4 \times 10^{6}}{0.1^2} =18\)
F = 18 N
So the correct answer is option 3.