Final answer:
The pointer on a vertically hung spring indicates the object's weight (option b) because the spring extends until its force matches the gravitational force acting on the object.
Step-by-step explanation:
When an object is attached to a spring that is hung vertically from the ceiling, the pointer indicates the object's weight. This is because the spring stretches until the force it exerts upwards (the elastic restoring force) matches the weight of the object due to gravity. The object's density, volume, or length are not directly measured by the displacement of the spring.
For instance, suppose you attach an object with mass m to a vertical spring originally at rest, and let it bounce up and down. If you release the object from rest at the spring's original rest length, the amplitude of the motion would then be the distance between the equilibrium position without the mass attached and the equilibrium position with the mass attached.
The weight of the object is calculated using the formula W = mg, where W is the weight, m is the mass, and g is the acceleration due to gravity, which is approximately 9.8 m/s2 on Earth's surface.