For an object at infinity, a concave mirror produces an image at its focus which is
A. Enlarge
B. Virtual
C. Erect
D. Real, inverted and diminished

Hint: If an object is at the focal point of the mirror, the image is formed at infinity. You can use a mirror formula to get the answer. Use the concept of concave mirror. Plane mirror itself gives a reflected image.

Complete Step-by-Step solution:
For an object at infinity, a concave mirror produces an image of its focus which is real, inverted and diminished. For all rays appear to come parallel to the principal axis.
As we know, the parallel rays coming from an object passes through the focus when reflected by the mirror.
So considering any parallel rays coming from an object, after reflection from a concave mirror passes through the focus of the mirror. Hence images formed at focus as point size.
As an image is formed due to converging rays in front of the mixture, therefore it is the real image.
Hence the formed image is real, inverted and diminished.

Additional information:
In a plane mirror, object distance is always equal to image distance. Size of both image and object are the same.
In a concave mirror, the object is placed away from the mirror then the image is inverted and at the point of focus. When the object is at focus point, image form is at infinity.
In convex mirrors, image forms are virtual, erect, and diminished irrespective of distance of object.
Correct option is (D)

Note: To calculate image position and feature, students can use a mirror formula. Mirror formula is applicable for both the mirrors, concave and convex mirror. Have a clear image for enlarge, virtual, effect, real, inverted, and diminish.