The potential of an object is due to all the charges present in the object, at its surface or outside. Thus, for V to be non-zero, charge must be present in on the surface or inside the body (here, if a cavity is present). Thus, option (a) is correct.

This point also confirms that option (c) is incorrect.

All the charge in a closed conductor resides on its outer surface and no charge can be present inside the body of the conductor. Thus, option (b) is correct.

Option (d) is also incorrect because for a close charged conductor, charges always lie on the outer surface of the conductor and yet it has a non-zero potential.