(a) (i) Pole is the centre of the reflecting surface of the mirror.(ii) Centre of curvature is the centre of the hollow sphere of which the reflecting surface of mirror is a part.(iii) Principal axis is the straight line passing through the pole and the centre of curvature of a spherical mirror.(iv) Principal focus: Incident rays parallel to principal axis, after reflection, either converge to or appear to diverge from a fixed point on the principal axis called principal focus of a spherical mirror.(c) $M$ is a concave mirror and the image is virtual.