Huygens’ principle allows us to understand and determine.

The correct answer is Option (4) → The propagation of wavefronts

Huygens’ principle is a geometrical construction: knowing the shape of a wavefront at one instant, it gives the shape of the wavefront at a later instant, using secondary wavelets from every point of the wavefront.

NCERT (2026-27):
Huygens Principle – “… Thus, Huygens principle is essentially a geometrical construction, which given the shape of the wafefront at any time allows us to determine the shape of the wavefront at a later time.”