(a) For head lights of a car, we use a concave mirror. The light source is held at the focus of the mirror. On reflection, a strong parallel beam of light emerges.
A convex mirror is used as side rear view mirror, because its field or view is larger, and it forms virtual, erect and diminished images of objects behind.
(c) For solar furnace, we use a concave mirror. Light from the sun, on reflection from the mirror, is concentrated at the focus of the mirror, producing heat.