Explanation :
Convex mirror is a spherical mirror in which the reflecting surface bulges towards the light source. There are only two cases for the image formation in a convex mirror.
(1) When an object is placed at infinity, Image will form at the focus behind the mirror. A virtual, erect and point sized image is formed.
(2) When an object is placed anywhere on the principal axis, the image will form between P and F behind the mirror. This forms a virtual, erect and diminished image.
Hence, correct option is ''anywhere on the principal axis''