What is Mechanical Equilibrium?
A.When no mechanical work is done by one part of the system on any other part of the system.
B.When no chemical work is done by one part of the system on any other part of the system.
C.When mechanical parts are done by one part of any other part of the system.
D.None of these

Hint: Mechanical Equilibrium is an equilibrium in which there are no unbalanced forces acting on any part of the system or on the system as a whole.

Complete step by step answer:
Equilibrium is defined as a stable situation or a state of balance where no changes are occurring and where opposing forces cancel each other out.
When no mechanical work is done by one part of the system on any other part of the system i.e. mechanical properties are uniform throughout the system, a system is said to be in mechanical equilibrium.
Also, a system is said to be in mechanical equilibrium when there is uniform pressure throughout the system.
We can also say that a particle is said to be in mechanical equilibrium if the net force acting on that particle is zero.
Example of Mechanical Equilibrium: Suppose a person is pressing a spring to a defined point. He or she can push it to an arbitrary point and hold it there, at which point the compressive load and the spring reaction are equal. In the above example, the system is in mechanical equilibrium.
Therefore, option B, C and D are incorrect options.
So, the correct option is option A, i.e. when no mechanical work is done by one part of the system on any other part of the system.

Note:
Mechanical Equilibrium is further of two types:
Static Equilibrium: Equilibrium where bodies are at rest is known as static equilibrium.
Dynamic Equilibrium: Equilibrium where bodies are moving at constant velocity known as dynamic equilibrium.
In both cases, the net force acting on them is zero.