Flow of fluid takes place due to its deformation under shear force.

Fluid flow is the movement of a fluid from one place to another. It occurs due to the deformation of the fluid under the application of a shear force. The four options given in the question are different properties of fluids, but only the option 'D' - deformation under shear force - is directly related to the flow of fluid. Let's understand why this is the correct answer in more detail.

Fluid Flow and Shear Force:
When a fluid is subjected to a shear force, it causes the fluid to deform and flow. Shear force refers to the force that acts parallel to a surface, causing one layer of fluid to move relative to another layer. This leads to a continuous flow of fluid.

Deformation of Fluid:
When a shear force is applied to a fluid, the fluid particles undergo deformation. The deformation can be visualized as the movement of fluid particles along layers or planes. As a result of this deformation, the fluid starts to flow.

Importance of Viscosity, Compressibility, and Surface Tension:
Although viscosity, compressibility, and surface tension are properties of fluids, they are not the primary factors responsible for the flow of fluid. Here's a brief explanation of each property:

- Viscosity: Viscosity refers to the internal resistance of a fluid to flow. It determines the fluid's ability to resist shear and deformation. While viscosity affects the rate of fluid flow, it is not the main cause of fluid flow.

- Compressibility: Compressibility refers to the ability of a fluid to be compressed under the application of pressure. In most practical situations, fluids are considered incompressible, meaning their volume does not change significantly with pressure. Compressibility does not directly cause fluid flow.

- Surface Tension: Surface tension refers to the cohesive forces between the molecules at the surface of a liquid. It causes the liquid to minimize its surface area and form a distinct boundary. While surface tension plays a role in capillary action and some fluid behaviors, it is not directly responsible for the flow of fluid.

Conclusion:
The correct answer is option 'D' - deformation under shear force. When a fluid is subjected to a shear force, it deforms, and this deformation leads to the flow of fluid. Viscosity, compressibility, and surface tension are important properties of fluids, but they do not directly cause fluid flow like deformation under shear force does.