Huygens' principle is applicable to

Which of the following generates a plane wavefront?

What is the shape of the wavefront on Earth for sunlight?

On a hot summer night,the refractive index of air is smallest near the ground and increases with height from the ground. When a light beam is directed horizontally,the Huygens' principle leads us to conclude that as it travels,the light beam

Explain the refraction of a plane wavefront by a thin convex lens.

Given below are two statements:
Statement-$I$: $A$ plane wave after passing through a prism remains a plane wave, but passing through a small pinhole may become a spherical wave.
Statement-$II$: The curvature of a spherical wave emerging from a slit will increase for increasing slit width.
In the light of the above statements, choose the correct answer from the options given below.