The body is said to be in the state of stable equilibrium when the centre of gravity of a body lies below the point of suspension or support. When the centre of gravity of a body lies above the point of suspension or support, it is said to be in the state of unstable equilibrium. Now, as the ball is at the top of the rod, is slightly disturbed, the centre of gravity is lowered down and line of action of its weight lies outside the base of the rod due to which a torque start acting on the ball. As a result the ball fell down. When the centre of gravity of a body lies on the point of suspension or support, it is said to be in state of neutral equilibrium.
Hence, the correct answer is "unstable equilibrium".