A

Step 1: Understanding the concept of focal length in mirrors:
The focal length ($f$) of a mirror is defined as the distance from the mirror's surface to its focal point.
Step 2: Plane mirrors have specific properties:
Plane mirrors do not converge or diverge light; they reflect light in a way that the reflection appears to be coming from a point behind the mirror.
Step 3: For a plane mirror, the focal point is considered to be at infinity. This is because the light rays that are parallel to the principal axis do not converge after reflection but appear to diverge from a point infinitely far behind the mirror.
Conclusion: Therefore, the focal length of a plane mirror is indeed considered to be infinity.