Question

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

Answer 1

Matter has mainly three states: solid, liquid, or gas. Solid matter is composed of tightly packed particles. Liquids and gases are composed of loosely packed particles and are also termed as fluids. As because of its loosely packed particles they could flow easily. Also other states of matter as plasma state and BEC i.e. Bose-Einstein Condensate.

Complete answer:
In the study of physics, a fluid is nothing but a type of substance that continuously flows when a shear stress, or external force is applied. They are phases of matter and matter can have states - liquids, gases and plasmas. They are matter which has zero shear modulus that means substances that cannot resist no shearing force applied to them.

Though the term "fluid" includes both the liquid and gas phases, commonly known as "fluid" is often used. Fluids display properties such as:
-Lack of resistance to permanent deformation, resisting only relative rates of deformation in a dissipative, frictional manner, and
-Their ability to flow (or the ability to take on the shape of any container).

These properties are mainly a function of inability to bear a shear stress in static equilibrium. In opposite to that, solids respond to shear either by a spring-like restoring force, which means that any type of deformations are reversible, or they require a certain initial stress before they deform synonymously with "liquid".

Hence Option C is the correct answer.

Note: A fluid is a type of substance that is deficient of rigidity; it cannot resist force when it is applied to it. Fluids come along for liquids. All the liquids that exist are fluid but all fluids are not liquids. Option A and B may be confusing but should be read carefully. Questions might be asked related to the difference between liquid and fluids.