Mirror: Convex mirror

Explanation:
A convex mirror is the only type of mirror that forms a virtual image. Here is a detailed explanation:

1. Concave mirror:
A concave mirror is a mirror with a curved surface that curves inward. This type of mirror can form both real and virtual images depending on the position of the object relative to the mirror.

- Real image: When the object is placed beyond the focal point of the concave mirror, a real image is formed. A real image is formed when the light rays converge and can be projected onto a screen.
- Virtual image: When the object is placed between the mirror and the focal point, a virtual image is formed. A virtual image is formed when the light rays appear to diverge from a point behind the mirror.

Since a concave mirror can form both real and virtual images, it does not fit the criteria of forming only virtual images.

2. Plane mirror:
A plane mirror is a flat mirror with a reflective surface. It forms virtual images by reflecting light rays. However, the virtual image formed by a plane mirror is laterally inverted, meaning it is a mirror image of the object but not inverted.

Since a plane mirror can form virtual images, it does not fit the criteria of forming only virtual images.

3. Convex mirror:
A convex mirror is a mirror with a curved surface that curves outward. This type of mirror always forms virtual images, regardless of the position of the object.

- Virtual image: When the object is placed anywhere in front of a convex mirror, the reflected light rays appear to diverge from a point behind the mirror. The virtual image formed by a convex mirror is always upright and smaller than the actual object.

Since a convex mirror forms only virtual images, it is the correct answer to the question.

In conclusion, among the given options, the only mirror that forms a virtual image is a convex mirror.