Question

Why plane mirror forms an erect image?

Answer 1

plane mirror forms an virtual image that means it forms an image befind the mirror. if you see the figure above shown in previous answer . image is formed behind the mirror at same distance and same height.
that's a point object A and so plane mirror formed a point image A' at same height and at same distance behind the mirrror due to its flat surface. if object is like a person then each point on person's body will have a image at same distance and same height behind plane or flat mirror.
that's why we see exact upright image of ourselves in the plane mirror.