Two equal and similar charges are placed along a straight line. A third charge equal but opposite to the other two is place midway between the two. The system will be in : 

non-equilibrium

The correct answer is Option 2: non-equilibrium

For a system to be in equilibrium, the net force on every charge must be zero. While the central charge is balanced, the outer charges are not.

• At the Center: The midway charge ($-q$) experiences equal and opposite attractive forces from both outer charges, so it is balanced.

• At the Ends: An outer charge ($+q$) experiences an inward attraction from the center ($F_1 \propto \frac{1}{r^2}$) and an outward repulsion from the opposite end ($F_2 \propto \frac{1}{4r^2}$).

Because the inward attraction is 4 times stronger than the outward repulsion ($F_1 \neq F_2$), the outer charges will collapse inward. The system can only be in equilibrium if the central charge is exactly $-\frac{q}{4}$. Since it is $-q$, the system is in non-equilibrium.