The correct answer is option 1, which states that the radius of curvature of a plane mirror is zero.

To understand this concept, we need to first understand what the radius of curvature means. In simple terms, it refers to the distance between the center of curvature and the mirror surface. It is typically used to describe curved mirrors, such as concave or convex mirrors.

However, a plane mirror is different from curved mirrors. It is a flat and smooth surface that does not have any curvature. Therefore, a plane mirror does not have a radius of curvature, as it does not have a center of curvature.

The reflection in a plane mirror appears to come from behind the mirror, creating a virtual image. The mirror simply reflects light without bending or focusing it in any way. Since there is no curvature in a plane mirror, the radius of curvature is zero.

Therefore, option 1 is correct.