A mirror forms a virtual image of a real object
A. It must be a convex mirror
B. It must be concave mirror
C. It must be a plane mirror
D. It may be plane or convex mirror

Hint: Recall the structures of concave mirror and convex mirror. The real image is formed if the reflected rays from the mirror can be focused on the screen. The virtual image can be formed by back tracing the reflected rays from the mirror.

Complete step by step answer:
To answer this question we first need to understand the types of mirrors. There are two types of mirrors: plane mirror and spherical mirror. We know that a spherical mirror also has two types: concave mirror and convex mirror.

We know that if the image of the real object can be formed on the screen, it is known as real image and if the image cannot be focused on the screen, it is known as virtual image.

We know that the rays reflected from the plane mirror never intersect at a certain point. Therefore, the real image of the object cannot be obtained from a plane mirror. So, the image formed by the plane mirror is always a virtual image.

When the outer surface of the spherical mirror is painted, the inner surface reflects light rays. We know that such a type of lens is known as a concave mirror. When the inner surface of the spherical mirror is painted then it is known as convex mirror. In the convex mirror, the reflecting surface is the outer surface. Therefore, the rays reflected from the convex mirror diverge in all directions. So, we cannot form the real image of the object using a convex mirror as the image formed by the convex mirror is focused by back tracing the reflected rays from the outer surface. The image formed by back tracing the reflected rays is always a virtual image.

Therefore, the virtual image can be formed by plane mirror and convex mirror.

We know that the image formed by the concave mirror may be real or virtual depending upon the distance of the object from the mirror. Therefore, the option (B) is incorrect.

So, the correct answer is “Option D”.

Note:
Students should know the characteristics of plane mirror, convex mirror and concave mirror. Also, the structures of convex and concave mirrors should be known to answer this type of questions. Note that concave mirror and convex lens forms both virtual and real images of the object depending upon the position from mirror or lens.