Which of the following is a type of mirage, or an optical illusion caused by atmospheric conditions?
Considering Fujita scale, Kuiper belt, Fata Morgana and Eiffel Tower, the answer is: Fata Morgana.
A Fata Morgana is an optical illusion, usually seen while sailing on a large body of water. Light and atmospheric conditions can bend light so that objects on the horizon appear to be up in the air, or much larger than they are. Sailors have been baffled by this phenomenon for millennia.
Fata Morgana is an incredibly complex mirage that occurs in very particular atmospheric conditions. More specifically, it is your brain assuming that when light hits your eyes, it arrived there in a straight path between you and the object reflecting the light. So if light is bent on its way toward you, your brain will think the object is where it would be if the light's path was straight. This is why when you are looking down on a surface of water, objects under the surface will appear to be in a different spot than they actually are. The human brain doesn't automatically account for the refraction.
In the case of ghost ships and mind-bending floating cities, the answer is not quite as mythical as the Fata Morgana's origins in fairy lore. In reality, the light reflecting from a distant object such as a ship is bent downward as it passes through the colder, denser air near the surface of the ocean, and the image is perceived further up in the air than it really is.
This works to similar effect in dry environments. In deserts, the absence of water bends the light up toward your eyes so that you perceive things as further down they actually are (ie the mirage of the forever-unreachable oasis).
Check out a few photos of the phenomenon below.