An image formed by a plane mirror is :

• Virtual images are the images formed when the light rays appear to meet or come from a point. 
• Virtual images cannot be obtained on the screen.
• Images formed in a plane mirror are virtual as they appear to form behind the mirror and cannot be obtained on the screen.
• Images formed in a plane mirror are also of the same size as the object.

Therefore, 'Virtual, formed behind the mirror and of the same size as the object' is the correct response.