Concept: Total internal reflection occurs when light travelling in a denser medium strikes a boundary at an angle greater than the critical angle and is reflected entirely back.Explanation: Optical fibres use this principle to keep light trapped inside the core of the fibre. The light repeatedly undergoes total internal reflection along the fibre wall. This allows light signals to travel long distances with very little loss of energy. Applications include internet cables, medical endoscopes, and telecommunication systems.Answer: D. fiber optics