(a) Let us considera Gaussian surface that is lying wholly within a conductor and enclosing the cavity. The electric field intensity E inside the charged conductor is zero. Let q is the charge inside the conductor and is Eo the permittivity of free space.

According to Gauss's law, Flux, $\mathrm{\varnothing }=E\cdot ds=\frac{q}{{ϵ}_0}$

Here,

$E=0\Rightarrow \frac{q}{{ϵ}_0}=0\Rightarrow q=0[\text{ as }{ϵ}_0\ne 0]$

Therefore, charge inside the conductor is zero.

The entire charge Q appears on the outer surface of the conductor.

(b) The outer surface of conductor A has a charge of amount Q. Another conductor B having charge tq is kept inside conductor A and it is insulated from A. Hence, a charge of the amount -q will be induced in the inner surface of conductor A and +q is induced on the outer surface of conductor A. Therefore, the total charge on the outer surface of conductor A is Q + q.

(c) A sensitive instrument can be environment by enclosing it fully inside a metallic surface. A closed metallic body acts as shielded from the strong electrostatic field in its electrostatic shield.