Final answer:
Both assertion A and reason R are true; however, reason R is not the complete explanation for assertion A as there are other phenomena, like induction, that can cause the electroscope leaves to move.
Step-by-step explanation:
Assertion (A): An electroscope can be used to find the presence of charge on a body.
Reason (R): It is based on the principle: Like charges repel each other.
The answer to the student's question is Option B: Both A and R are true but R is not the correct explanation of A. The electroscope does indeed detect the presence of charge, and its operation is based on the principle that like charges repel each other. However, this is not the only principle at play. An electroscope can also identify the presence of charge by induction, where a charged object brought near the electroscope causes the leaves to move without direct contact. This phenomenon involves both repulsion and attraction (opposite charges attract) of charges.