Fluids are
A) The substances which are lighter than air
B) The substances which let substances float on them.
C) The substances which can flow easily
D) Liquids

Hint
To answer this question, we have to analyse the properties of the fluids which differentiate it from the other substances. We need to check each option given in the question based on these properties, and eliminate all the incorrect options.

Complete step by step answer
We know that the fluids are the substances which tend to deform continuously on the application of the external force. It is this continuous deformation of fluids which differentiate them from the other substances. For example, in the case of solids, when the external force is applied, they show only a limited deformation. Also there is a restoration of the original dimensions. But in a fluid, the original dimensions are never restored.
It is found that both the gases and the liquids show the properties of a fluid. So they both are considered as fluids. So not only liquids, but gases also come under the category of the fluids.
Hence, option D is incorrect.
There are many gases in nature, which may or may not be lighter than air, but still are fluids. So, the density of air is not a differentiating parameter between fluids and other substances.
Hence, option A is also incorrect.
As we know that for a substance to float in a fluid, its density should be less than the fluid. So, a substance may or may not float in a fluid, depending upon its density.
Hence, option B is also incorrect.
So, the only option remaining is option C, which states that fluids are the substances which can flow easily, which is very much clear from its definition. On the application of external force, they deform continuously. This continuous deformation is nothing but the flow.
Hence, the correct answer is option C.

Note
There should be no confusion regarding option B, which states that a fluid is a substance which lets other substances float on it. The incorrectness of this statement is justified on the basis of the law of floatation, which puts the condition on the density of a substance, to be less than that of the fluid.