Concept:A wavefront is the surface formed by all points having the same phase.Explanation:For a point source in a homogeneous isotropic medium, light travels equally in all directions.After time t, all points at distance r=vt from the source have the same phase.These points lie on a sphere centred at the source.Hence, the wavefront is spherical.Answer:The wavefront is spherical, so the correct option is D.