Q.What is electrostatic shielding? Mention one use of it.

The core concept here is electrostatic shielding: the field inside the cavity of a conductor depends only on the charge enclosed within that cavity, never on anything outside the conductor — however strong, however close. This is exactly the property that makes a Faraday cage work, and NCERT states it explicitly for the cavity-in-a-conductor case.

Why the field cannot reach $q_2$

1. Shell 2 is a conductor in electrostatic equilibrium, so the field is zero everywhere within the metal of shell 2 itself.

2. The charge on the inner surface of shell 2 is fixed by $q_2$ alone. Because $q_2$ sits exactly at the centre of the spherical cavity, the induced charge on the inner wall must be a uniform layer of total charge $-q_2$ — this is the only distribution consistent with (a) the field being zero in the conductor bulk, and (b) $q_2$ being at the geometric centre. This uniform layer is completely determined by $q_2$'s position; it has nothing to do with $q_1$ or shell 1.

3. Whatever the external field from shell 1 does, it can only rearrange the outer surface charge of shell 2 (making it non-uniform, so that shell 2's outer surface, seen from far away, is no longer a perfect point-charge equivalent). This outer-surface rearrangement is exactly what "soaks up" the external field — it cannot communicate anything back through the conducting bulk to the inner surface or the cavity. …