Standoff distance refers to the space maintained between a potential explosive threat (such as a vehicle-borne improvised explosive device, or VBIED) and the target asset. Increasing standoff distance significantly reduces the impact of a blast by leveraging the inverse-square law, which governs blast pressure dissipation over distance.
A (Blast radius) is the area affected by an explosion.
B (Impact area) refers to where a blast or projectile strikes.
C (Clear zone) is used in security but does not specifically refer to bomb offset.
References:
PSP Study Guide - Blast Mitigation and Perimeter Design
POA Manual - Vehicle Threat Assessment and Standoff Principles