Huygens' principle of secondary wavelets may be used to

In the adjoining diagram, a wavefront $AB$ moving in air is incident on a plane glass surface $XY$. Its position $CD$ after refraction through a glass slab is shown along with the normals drawn at $A$ and $D$. The refractive index of glass with respect to air $(\mu = 1)$ will be equal to

Which of the following generates a plane wavefront?

What is the shape of the wavefront in each of the following cases:
$(a)$ Light diverging from a point source.
$(b)$ Light emerging out of a convex lens when a point source is placed at its focus.
$(c)$ The portion of the wavefront of light from a distant star intercepted by the Earth.

Carefully analyze the given diagrams and choose the correct option regarding the refraction of a plane wavefront by a thin convex lens:

Write the limitation of Huygen's principle.