Among the given options, which is the most appropriate mirror that is small in size, but can form a full virtual image of a tall building?
a convex mirror
To fit the whole building in a small mirror, the mirror has to form a diminished image.
A plane mirror which is small in size cannot form the complete image of a tall building.
Concave mirrors cannot always form diminished images.
However, convex mirrors always form a diminished image of the object irrespective of object position. Hence, it is the most appropriate mirror for this application.