Final answer:
A person falling over after being nudged represents an unstable equilibrium as they do not return to their initial position without additional effort.
Step-by-step explanation:
In the scenario where a person who is walking loses balance and falls when nudged, this illustrates an unstable equilibrium. A stable equilibrium is a state in which a system will return to the same position after a slight disturbance, while an unstable equilibrium is a condition where any small disturbance will move the system away from its initial position, leading to a fall or toppling over in the case of a human. Since a person falling over after losing balance does not return to the initial upright position without additional effort, this example does not represent stable equilibrium or neutral equilibrium. Therefore, the correct answer is B. Unstable equilibrium.