The correct answer is (a) concave.
A concave mirror is a curved mirror that bulges inward at the center. It reflects light rays in such a way that they converge at a point in front of the mirror. This makes concave mirrors useful for focusing light, such as in searchlights.
A convex mirror is a curved mirror that bulges outward at the center. It reflects light rays in such a way that they diverge, or spread out. This makes convex mirrors useful for providing a wider field of view, such as in rearview mirrors.
A plane mirror is a flat mirror. It reflects light rays in such a way that they bounce off the mirror at the same angle that they hit it. This makes plane mirrors useful for providing a true reflection of an object.
I hope this helps!