What type of image is formed by a plane mirror?

A plane mirror always forms a virtual image (at the back of the mirror).
A plane mirror always forms a virtual image behind the mirror. The image formed in a plane mirror is the same size as the object, and it is upright. 

Virtual images are formed in locations where light does not actually meet, but they appear to be located behind the mirror due to the reflection. Light does not pass through the image itself but appears to be coming from behind the mirror. 

The nature of the image formed by a plane mirror is virtual, upright, and located behind the mirror and the distance of the image behind the mirror is equal to the distance of the object in front of the mirror, measured from the center of the mirror along the principal axis.
 

Hence, the option "virtual and same size" is correct.