In which of the following fluid the shear stress is directly proportional to the rate of shear strain? 

Ideal fluid

Fluids which don’t have viscosity and are incompressible are termed as an ideal fluid such fluid do not offer shear resistance i.e no resistance is encountered as the fluid moves.

Real fluid

Fluids which do possess viscosity are termed as real fluids. These fluids always offer shear resistance i.e. Certain amount of resistance is always offered by these fluids as they move.

Newtonian fluids

Newtonian fluids (a real fluid) defined as fluids for which the shear stress is linearly proportional to the shear strain rate

Newtonian fluids are analogous to elastic solids (Hooke’s law: stress proportional to strain)

Any common fluids, such as air and other gases, water, kerosene, gasoline, and other oil-based liquids, are Newtonian fluids

\(\tau = \mu \frac{{du}}{{dy}}\) where μ is shear viscosity of the fluid

A fluid whose viscosity does not change with the rate of deformation or shear strain is known as Newtonian fluid.

Non-Newtonian fluid

Fluids for which the shear stress is not linearly related to the shear strain rate are called non-Newtonian fluids examples include slurries and colloidal suspensions, polymer solutions, blood, paste, and cake batter

In non-Newtonian, the viscosity is dependent on shear rate (Shear Thinning or Thickening) .