$(a)$ Let us consider a 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 $\epsilon_{0}$ the permittivity of free space. According to Gauss's law, Flux,

$\phi=E . d s=\frac{q}{\epsilon_{0}}$

Here, $E =0 \Rightarrow \frac{q}{\epsilon_{0}}=0 \Rightarrow q=0$

$\left[\text { as } \epsilon_{0} \neq 0\right]$

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 $+ q$ is kept inside conductor $A$ and it is insulated from $A$. Hence, a charge of amount $- q$ will be induced in the inner surface of conductor $A$ and $+q$ is induced on the outer surface of conductor $A$. Therefore, total charge on the outer surface of conductor $A$ is $Q+q$

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