The image of an optical pin placed at the principal focus of a concave mirror will be formed
Answer Details
When an object is placed at the principal focus of a concave mirror, the reflected rays will be parallel to each other. This happens because the light rays coming from the object get refracted by the mirror in such a way that they all converge at the principal focus of the mirror. Therefore, the image of the object will be formed at infinity.
So, the correct option is "at infinity".