In the case of a plane mirror, the focal length is considered to be infinite. This means that the light rays coming from an object that gets reflected from a plane mirror appear to converge at a point that is at an infinite distance behind the mirror.
The spherical mirror formula, which relates the object distance (u), image distance (v), and focal length (f), is applicable to curved mirrors such as convex and concave mirrors. In the formula, the focal length is a significant parameter that helps determine the characteristics of the image formed by the mirror.
Therefore, in the context of the question, when the focal length is said to go to infinity, it implies that the mirror is a plane mirror. With an infinite focal length, the mirror behaves like a flat surface, and the image formed is virtual, upright, and the same size as the object.
Hence, option 1 is correct in stating that the spherical mirror formula can be applied to a plane mirror when the focal length goes to infinity.