A substance that continuously deforms under an applied shear stress is called a
A. solid
B. fluid
C. plastic
D. None of these

Hint: Shear stress is defined as the ratio of force applied to deform the material and the cross sectional area of the material. If the material requires a large amount of force to deform it then the shear stress will be more. So if the force required to deform the material is less, the shear stress will be less and the material deforms continuously with the applied force.

Complete step by step answer:
We are given three substances and we have to find which of these substances continuously deforms under an applied shear stress.
Shear stress is the force used to deform a substance or a material. Shear stress is coplanar (parallel) with the cross section of the material.
Shear stress is a vector quantity and is defined as the ratio of the force applied and the cross sectional area of the material.
If the force required to deform the material is less, the shear stress will be less and the material deforms continuously with the applied force.
First option given is solid. The atoms in the solids are tightly packed and therefore large force is required to deform a solid.
Second option is fluid. The atoms are loosely packed in fluids and less force is required to deform the fluids. They continuously deform under an applied shear stress.
Third option is plastic. Plastic is a solid object. So, great force is required to deform plastic substances.
So, a substance that continuously deforms under an applied shear stress is called a fluid.


So, the correct answer is “Option B”.

Note:
Another approach.
Shear stress can be defined as the product of shear strain and shear modulus. If a substance has very low shear modulus then the substance continually deforms under an applied shear stress.
The substance with least shear modulus will deform continuously.
Solids have high shear modulus as they require force to be deformed.
Fluids are substances that have zero shear modulus.
Plastic is a solid object.
So, the fluid is the substance which deforms continuously under applied shear stress.