Concept:When an object is placed at the principal focus of a concave mirror, the reflected rays become parallel and the image forms at infinity.
Explanation:For a concave mirror, an object at the principal focus
F sends rays that reflect parallel to the principal axis.
Parallel rays never converge at a finite point.
They appear to meet only at infinity.
Therefore, the image is located at infinity.
This is a standard result for concave mirrors when
u=f.
Answer:C. at infinity