Which of the following phenomena is used for propagation of light in…

The correct answer is Option (4) → Total internal reflection

An optical fibre has a glass core surrounded by a cladding of slightly lower refractive index. Light entering the core strikes the core–cladding boundary at an angle greater than the critical angle, so it is totally internally reflected again and again and travels along the fibre, even round bends, with very little loss.

NCERT (2026-27):
Total Internal Reflection in Nature and its Technological Applications – “… Optical fibres: Nowadays optical fibres are extensively used for transmitting audio and video signals through long distances. Optical fibres too make use of the phenomenon of total internal reflection. … When a signal in the form of light is directed at one end of the fibre at a suitable angle, it undergoes repeated total internal reflections along the length of the fibre and finally comes out at the other end (Fig. 9.14).”