A driving mirror on a car is never concave because:

As is common knowledge, using a mirror that provides its entirety in a small region of the mirror is a must when operating a motor vehicle. Concave, however, produces a picture of the distant object that is reversed.

A concave mirror always produces a true and upside-down image of the faraway object, which is inappropriate for the car.
Therefore, since a convex mirror always creates a smaller picture, we should utilise that instead of a concave mirror. The ability to observe the entire scene always assists the driver. Another name for it is a rearview mirror.
Because the concave mirror does not provide the whole field of vision, we are unable to use it.
So the correct answer is (A) The viewing area is too small.