Step 1: Analyze image formation by a Plane Mirror.
A plane mirror always forms a virtual, erect (upright), and same-sized image as the object. It does not form a diminished image.
Step 2: Analyze image formation by a Concave Mirror.
A concave mirror can form both real and virtual images, and these can be magnified, diminished, or same-sized depending on the object's position. However, when it forms a virtual image, that image is always erect and magnified, not diminished.
Step 3: Analyze image formation by a Convex Mirror.
A convex mirror always forms a virtual, erect (upright), and diminished image, regardless of the object's position. This is a characteristic property of convex mirrors.
Step 4: Compare with the given properties.
The problem states that the mirror forms a virtual and diminished image. Based on the analysis, only a convex mirror consistently forms a virtual and diminished image.
$$(1) Convex mirror$$