In which of the following cases do we obtain a spherical wave front ?
a) Sunlight focussed by a convex lens 
b) Light diverging from a straight slit
c) Light emitted by a point source in an isotropic medium
d) A parallel beam of light reflected from a plane mirror

 Spherical, 
The point source emits the spherical wavefront as the light diverges from a point source in all directions.

 Planar, 
The shape of the wavefront in case of a light emerging out of a convex lens when a point source is placed at its focus is a parallel grid.

 Planar, 
(A small area on the surface of a large sphere is nearly planar). The portion of the wavefront of light from a distant star intercepted by the Earth is a plane.