A mirage occurs because

A mirage occurs when light rays from a distant object appear to be emanating from the ground-based representation of the object. We are aware that all light waves, regardless of their origin or destination, pass through essentially the same layers of air. Each person is hunched over to a similar degree. The upward-directed rays will arrive lower on the item. We may ascertain the cause of the mirage occurrenceusing these ideas.

Have you ever driven on a scorching road or encountered a mirage, which is an illusion of the sight of water? Or have you noticed that items above and beyond a holy fire seem to be trembling? What causes this to occur?
As we ascend above, the refractive index likewise falls because cold air descends while hot air rises above.

Light refraction is somewhat influenced by the local atmosphere. The air near the heated surface of the road or desert, as well as the sacred fires, is hot and hence rarer than the air above it in both of the aforementioned situations. As we climb higher, the refractive index of the air continues to rise. According to the rules of refraction, in the first example above, the direction of distant light beams changes with time.
The illustration depicts the picture of the item inside the earth, from which it appears that light rays are emanating from a distant object. It's known as a mirage.

In the second illustration, a change in refractive index above the holy fire causes the direction of light rays emanating from things beyond the fire to alter. The items therefore seem to be moving.

By virtue of complete internal reflection, light rays travelling from a denser, colder medium to a rarer, hotter media—all travelling from the same distance or distant object—travel through approximately the same air layers and are bent over almost equally. As a result, rays entering the item from the top will arrive lower than those entering it from the bottom, and vice versa. By making it appear as though the distant sky picture is actually an oil pool of water serving as a mirror, the image is often upside down.

Hence a mirage occurs because the refractive index of the atmosphere increases with height.
So, the correct answer is “Option A”.