Which of the following is not considered a fluid?

The question asks which of the options is not considered a fluid. To determine this, it's essential to understand the definition of a fluid. A fluid is a substance that has no fixed shape and yields easily to external pressure; it can flow and take the shape of its container.

Oil, xenon gas, and mercury all exhibit these qualities. Oil is a liquid that flows freely and adapts to the shape of its container. Xenon gas, while in a gaseous state, also exhibits fluid characteristics because it can diffuse and occupy the volume of its container. Mercury is a liquid metal, recognized for its unique properties, but it is still classified as a fluid because it flows and has no fixed shape.

When the option states "None of these," it signifies that all listed substances are indeed considered fluids. Therefore, the correct understanding is that every option provided represents a fluid, making "None of these" the appropriate selection as the answer to the question posed.