The assertion suggests that the outer body of an aircraft is made of metal to protect passengers from lightning strikes. The reason provided is that the electric field inside a cavity enclosed by a conductor is zero.

This reasoning is based on the property of metallic conductors, where the electric field within a conductor's cavity is indeed zero. This phenomenon occurs because the charges on a conductor redistribute themselves to cancel any external electric fields within the conductor. Thus, if lightning were to strike an aircraft, the metal body would direct the electric charges along its surface, effectively shielding the interior where passengers are located.

Therefore, the assertion and the reason are both correct, and the reason correctly explains why the metal body of an aircraft shields passengers from lightning. This aligns with the principles of electromagnetic shielding. The key point is that the conductive nature of the aircraft’s metal body ensures no electric field permeates the interior, keeping passengers safe.