X is a mirror which has a smaller field of view than a plane mirror with the same aperture, and is a focusing mirror. X is
As you can see clearly, the concave mirror has a smaller field of view, whereas convex larger than the plane mirror. (all having the same aperture, or length exposed to light).
Now let us see what is the fate of parallel rays falling on these mirrors.
As is clear, it is the concave mirror which is capable of focusing (that is converging parallel rays after reflection).