D at the focus of the mirror.
From a concave mirror, if an object is at infinity, parallel light rays will converge at focus after reflection. As a result, the formed image is what the mirror is focused on.
When an object is at infinity from a concave mirror, the image formed is A at the centre of curvature of the mirror. B anywhere beyond the centre of curvature of the mirror. C between the centre of curvature and the focus of the mirror. D at the focus of the mirror