Assertion (A): The electric field inside a charged metallic sphere is zero.

Correct answers: (a) Both (A) and (R) are true and (R) is the correct explanation of (A) or (c) (A) is true, but (R) is false

Explanation

(A) is true. In electrostatic equilibrium the electric field inside a conductor is zero: any field would push the free electrons until the field was cancelled.

For a charged metal sphere, the extra charge spreads over its outer surface. A closed surface drawn inside the metal then encloses no charge, so by Gauss's law the field inside is zero. Read this way, (R) is true and explains (A), which gives (a).

The book gives (c). It reads the word "only" in (R) strictly: if a charge is placed inside a hollow in a conductor, an equal and opposite charge appears on the inner surface of the hollow, so charge is not always only on the outer surface.

The book's key is (c): it reads "only" in (R) strictly, since a charge placed inside a hollow conductor draws charge to the inner surface. For a charged metal sphere, (R) is true and explains (A), so (a) is also accepted.