Explanation:
A mirage is an optical illusion that occurs due to the bending of light rays. It is a phenomenon where distant objects appear to be shimmering, distorted, or displaced from their actual position. The correct answer is option 'D' because a mirage fits the description of an optical illusion.

Definition of a Mirage:
A mirage is a phenomenon that occurs when light rays bend due to the variation in air temperature. It usually happens in hot, desert-like environments where the ground is significantly heated. The bending of light causes an apparent displacement of objects, creating the illusion of water or reflections.

Causes of Mirage:
1. Temperature Gradient: Mirages occur due to the temperature gradient in the air. The air close to the ground is hotter than the air higher up. This temperature difference causes the light rays to bend as they pass through the layers of air with varying densities.

2. Total Internal Reflection: When light travels from one medium to another, it bends or refracts. In the case of a mirage, the temperature gradient causes the light rays to bend more than usual, leading to total internal reflection. This reflection creates the illusion of water or a shiny surface.

Types of Mirage:
1. Inferior Mirage: An inferior mirage is the most common type of mirage. It occurs when the air close to the ground is hotter than the air above. This causes the light rays to bend upwards, creating an image of objects below the actual position.

2. Superior Mirage: A superior mirage occurs when the air close to the ground is colder than the air above. In this case, the light rays bend downwards, creating an image of objects above their actual position. Superior mirages are often seen in cold Arctic regions.

Characteristics of a Mirage:
1. Shimmering Effect: Mirages create a shimmering effect, making the reflected image appear unstable or wavering.

2. Displacement: Mirages can displace the position of objects, making them appear higher or lower than their actual location.

3. Illusion of Water: One of the common characteristics of a mirage is the illusion of water. Due to the bending of light, distant objects may appear as if they are reflecting off a water surface.

4. Distance: Mirages often occur at a distance, particularly in desert areas where the ground is heated.

In conclusion, a mirage is an optical illusion that occurs due to the bending of light rays caused by a temperature gradient in the air. It creates the illusion of shimmering, displaced, or distorted objects, often resembling water or reflections.