(C)
A ray parallel to the principal axis, after reflection from a concave mirror, passes through the principal focus F. Hence, option (C) is correct.
Source: Chapter 9, Section 9.2.2 — Representation of Images Formed by Spherical Mirrors Using Ray Diagrams
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The first standard rule for ray diagrams of concave mirrors states: a ray parallel to the principal axis, after reflection, must pass through the principal focus F. Options (A), (B), and (D) show the reflected ray missing F or going in the wrong direction, violating this rule. Examiners specifically test this rule, so always remember: parallel ray → reflects through F (concave mirror).