The answer is A. Stable.
In a simple harmonic motion, the position of equilibrium is always stable. This means that if the system is displaced from its equilibrium position, it will tend to return to that position.
The reason for this is that the restoring force in a simple harmonic motion is always directed towards the equilibrium position. This force is proportional to the displacement from the equilibrium position, and so it becomes stronger the further the system is displaced from equilibrium. This means that the system will always tend to move back towards the equilibrium position.
An example of a simple harmonic motion is a mass on a spring. If the mass is displaced from its equilibrium position, it will experience a restoring force that will tend to return it to the equilibrium position. The mass will oscillate back and forth around the equilibrium position until it eventually comes to rest at that position.
The other options are incorrect. Option B, unstable, is incorrect because the system will not tend to return to the equilibrium position if it is displaced from that position. Option C, neutral, is incorrect because the system will not tend to move towards or away from the equilibrium position if it is displaced from that position. Option D, none of the above, is incorrect because the position of equilibrium is always stable in a simple harmonic motion.