Final answer:
The illusion of water on hot asphalt is a mirage, caused by the refraction of light rays due to the variation in the refractive index of air with temperature.
Step-by-step explanation:
The illusion of water on top of hot asphalt on a summer day is known as a mirage. Mirages occur when there is a hot layer of air near the ground causing the refractive index of air to decrease with higher temperatures.
This variation in refractive index causes light rays from distant objects to bend or refract as they pass through layers of air with different temperatures, creating the illusion of water. Among the given options, B. a mirage is the correct answer.