ANS:D - flows when even a small shear is applied to it.

Among the options provided, the correct statement that defines a fluid is: flows when even a small shear is applied to it. Here's why:

1. Flow under Shear: A fluid is defined by its ability to flow when subjected to an applied shear stress. This means that even a small force or stress applied tangentially to the fluid will cause it to deform continuously and flow.
2. Definition of a Fluid: A fluid is a substance that can flow and take the shape of its container. It includes both liquids and gases, and is characterized by its ability to deform continuously under an applied shear stress.

• Has to be kept in a closed container: This is not a defining characteristic of a fluid. A fluid can exist in open or closed containers, or in unbounded environments (like the atmosphere).
• Is almost incompressible: While some fluids (like liquids such as water) are nearly incompressible, compressibility alone does not define a fluid. Gases, for example, are compressible fluids.
• Has zero shear stress: Fluids do not have zero shear stress; rather, they flow in response to shear stress. Shear stress is a fundamental property of fluids that causes them to deform and flow.