(c) find the new position of a wavefront
(d) explain Snell's law.
EXPLANATION:
The Huygen's principle of secondary wavelets says that the points of a wavefront act as secondary sources of light emitting secondary wavelets. The line joining the points on the secondary wavelets that are in the same phase gives the new position of a wavefront. Hence the option (c) is true.
It can also be used to explain the Snell's law. Hence the option (d) is true. Options (a) and (b) are not true.