To determine the nature of the mirror based on the behavior of image formation, let's analyze the observations:

1. A mirror forms the image between the pole and the focus irrespective of the location of the object.
2. This is a characteristic behavior of a convex mirror.

Here's why a convex mirror fits the description given in the question:

• A convex mirror always forms a virtual, diminished image located between the pole and the focus. This occurs because the reflective surface bulges outward, causing the reflected rays to diverge. The perceived image is always behind the mirror due to the extended rays.

Let's examine the options:

• Plane mirror: Forms an image the same size as the object, located directly behind the mirror at a distance equal to the object's distance from the mirror. Thus, it cannot always form the image between the pole and the focus.
• Concave mirror of large focal length: Can form real or virtual images depending on the object's position. The real image formation occurs beyond focus for different object positions, unlike convex mirrors.
• Concave mirror of small focal length: Similarly, this can form real or virtual images, but real images can be formed beyond focus depending on the object's location.
• Convex mirror: The correct choice, as explained, aligns perfectly with the behavior described in the question.

Therefore, based on the nature of the image formation, the mirror in question is a convex mirror.