For an object at infinity, a concave mirror produces an image at its focus that is:
D. Real, inverted, and diminished.
When an object is placed at infinity (meaning the rays of light coming from the object are parallel), a concave mirror focuses those parallel rays of light onto its focal point. The image formed in this case is real, meaning the light rays converge at a point to form a real image that can be projected onto a screen. The image is also inverted, meaning it is upside down compared to the object. Additionally, the image is diminished or reduced in size compared to the object.