In which state does matter have the highest entropy?
A. Solid
B. Liquid
C. Gas
D. Equal in all

Hint: The second law of thermodynamics states that the entropy of the universe which includes the system and surrounding increases for an irreversible process.
It remains constant for a reversible process.
This law gives the idea of entropy as a physical property of a thermodynamic system.

Complete step-by-step answer: Entropy is defined as the measure of the extent of disorderness in a thermodynamic system.
It indicates the abnormality or absence of uniformness in the structure of a thermodynamic system.
This is denoted by ‘S’.
It is an extensive property as the measure of entropy or entropy change depends on the amount of matter existing in a thermodynamic system.
Entropy is a state function because its value does not rely on the path by which the system obtained a particular state.
It relies on only the initial and final state of the system.
For example, when we leave the paper on fire these two things form an isolated system.
If we start heating the paper, the paper will start burning and will slowly turn into a gaseous state.
In this case, the entropy rises.
This is because the paper was in a solid state where all its constituent particles are tightly packed and maintained a uniform order.
There is no irregularity or abnormality in the structure of the solid.
But when there is a change of state, the gaseous state is slowly forming.
In a gaseous state, the constituent particles move randomly.
We know that for gas, the size of the constituent particles is negligible as compared to the volume of the container in which the gas is present.
So, these particles move randomly and are far apart from each other.
The degree of randomness is highest in the gaseous state.
In the liquid state, the constituent particles also move randomly but the degree of randomness is lower than that of the gaseous state.

So, option C is correct.

Note: All the spontaneous processes are irreversible, so the entropy of the universe is expanding.
Entropy is associated with spontaneity.
The higher the spontaneity in a system, the higher the degree of disorderness which will result in higher entropy.