The correct answer to this question is option 4: Total internal reflection of light. The phenomenon of mirage occurs when light traveling from a distant object passes through layers of air with different densities, causing the light to change its direction. This change in direction creates an illusion of the object being located at a different position than its actual location.

Total internal reflection is the phenomenon where light waves are completely reflected back into the medium they originated from, rather than being transmitted through a different medium. In the case of a mirage, light undergoes total internal reflection when it passes from a hotter layer of air near the ground to a cooler layer of air above it.

Options 1, 2, and 3 are not correct explanations for the phenomenon of mirage. Polarisation of light refers to the alignment of light waves in a specific direction. Dispersion of light refers to the splitting of white light into its component colors. Diffraction of light refers to the bending of light waves around obstacles. While these phenomena may occur in different contexts, they are not directly responsible for the occurrence of mirages.

In conclusion, the correct explanation for the phenomenon of mirage is option 4: Total internal reflection of light.