Concept:The gold-leaf electroscope detects electric charge, and the amount of divergence of its leaves shows the relative magnitude of the charge present.
Explanation:When a charged body touches the metal cap of a gold-leaf electroscope, the charge flows down to the gold leaves.
The gold leaves then receive similar charges and repel each other, causing them to diverge.
The larger the charge transferred to the electroscope, the greater is the force of repulsion, and therefore the wider the leaves spread apart.
To compare the magnitudes of charges on two bodies, bring each body in contact with the electroscope one after the other and observe the divergence of the leaves.
The body that produces a larger divergence possesses a greater magnitude of charge, while the body that produces a smaller divergence has a lesser amount of charge.
Thus, the gold-leaf electroscope serves as a relative indicator for comparing charges.
The other instruments mentioned do not serve this purpose of charge comparison.
Answer:C. Gold-leaf electroscope