The correct answer is option 'C', Mirage.

Mirage does not belong to the group because it is not a permanent or physical entity like the other options. Let's analyze each option in detail to understand why Mirage is the odd one out:

a) Oasis:
An oasis is a fertile area in a desert where water is present, typically surrounded by vegetation. It is a physical and geographical feature that exists in reality. Oasis is associated with the presence of water in an arid environment.

b) Fountain:
A fountain is a structure that jets water into the air for decorative or practical purposes. It is a man-made installation that usually consists of a basin, jets, and pump system. Fountains are commonly found in parks, gardens, and public spaces.

c) Mirage:
A mirage is an optical illusion caused by atmospheric conditions, typically in deserts or hot asphalt roads. It occurs when light rays are bent or refracted by layers of air at different temperatures, creating a false image that appears as water, objects, or landscapes. Mirages are not physical or tangible; they are temporary and disappear when observed closely.

d) Pond:
A pond is a small body of still water, usually shallow and often artificially created. It is typically smaller than a lake and can support various aquatic plants and animals. Ponds can be natural or man-made and are commonly found in residential areas or parks.

e) Lake:
A lake is a large body of water surrounded by land. It is typically formed by geological processes such as tectonic activity, volcanic activity, or glaciation. Lakes are permanent features and can vary in size and depth. They provide habitats for various plants and animals and are often used for recreational activities like swimming and boating.

Therefore, among the given options, Mirage is the odd one out because it is not a physical feature like Oasis, Fountain, Pond, or Lake. It is an optical illusion caused by atmospheric conditions.