Step 1: Understanding the Concept:
At the midpoint (origin), the forces from both charges cancel. For displacement along \(x\)-axis, it is unstable equilibrium; for displacement along \(y\)-axis, restoring force acts.
Step 2: Detailed Explanation:
If displaced along \(y\)-axis by small \(y\), the net force is directed back towards origin (restoring), producing SHM. Along \(x\)-axis, displacement leads to a repulsive force away from origin (unstable).
Step 3: Final Answer:
The particle executes SHM along the \(y\)-axis.