Correct answer: (a) Total internal reflection
Explanation
- A
Total internal reflection
(a) Total internal reflection. Light in the fibre core hits the cladding at an angle greater than the critical angle and stays guided. (a) Official key.
- B
Diffraction
(b) Diffraction. Bending around obstacles or through slits; not the guiding mechanism of a telecom fibre.
- C
Scattering
(c) Scattering. Rayleigh scattering causes loss in fibre; it is not the useful transmission principle.
- D
Refraction
(d) Refraction. Needed to define the critical angle, but the energy stays in the core because of TIR, not ordinary refraction out of the core.
Summary. Official key is (a). Optical-fibre transmission uses total internal reflection at the core–cladding boundary. Diffraction, scattering and ordinary refraction are other optics words, not the guide.