Final answer:
The elastic force is responsible for restoring a vertical spring to its equilibrium position, as it acts to oppose the deformation caused by the displacement from equilibrium.
Step-by-step explanation:
The force responsible for restoring a massless vertical spring to its equilibrium position is the elastic force. This force is inherently related to the properties of the spring itself and is a reactive force that resists deformation. When the spring is displaced from its equilibrium position, either by compression or extension, the elastic force acts to bring the spring back to this position, opposing the displacement. This follows Hooke's Law, which states that the force exerted by a spring is directly proportional to the displacement, provided the limit of elasticity is not exceeded.
According to the information provided, the restoring force is the force which brings an object back to its equilibrium position (option d). When a mass is attached to a spring and the spring stretches, the elastic force of the spring provides the restoring force, counteracting the weight of the mass, and allowing the spring to return to equilibrium when the mass is removed. This is illustrated in Figure 5.5 and Figure 16.3, where the restoring force causes an object to oscillate around its equilibrium position in the absence of damping forces, such as friction.