Which of the following forms a virtual and erect image of the same size for all positions of the object?

The correct option is B

Plane mirror

The explanation for the correct option:

In the case of option (B),

1. For all the positions of the plane mirror, it forms only a virtual and erect image of the same size
2. Given below is a diagram of an image formed by the plane mirror
3. In the diagram shown below, $h_o=h_i$

The explanation for the incorrect options:

1. In the case of option A, a convex lens can have a diminished inverted image, enlarged inverted image, or enlarged erect image.
2. In the case of option C, a convex mirror forms a diminished image not an image of the same size.
3. In the case of option D, a concave mirror can form both real and virtual images.

Therefore, option B is the correct answer.