Answer:
B. It is reflected back through the focus.
Step-by-step explanation:
Principal axis is normal to the center of the mirror, and vertex is the point at which the principal axis touches the surface of the mirror.
When light rays strike a mirror ( example concave mirror), parallel to its principal axis, these rays will be reflected by the mirror such that all the rays will pass through the same point on the principal axis. The point on the principal axis through which these rays will pass through is known as focus.
Therefore, light ray that hits a mirror at its vertex, will be reflected back through the focus.
Thus, the correct answer is "B. It is reflected back through the focus".