For a spontaneous process, the correct statement is:
A. Entropy of the system always increases
B. Free energy of the system always increases
C. Total entropy change is always negative
D. Total entropy change is always positive

Hint: We must have to know that entropy is basically a very integral part of the second law of thermodynamics and also we call the second law of thermodynamics is the law of entropy. Entropy is a very abstract term. We need to remember that entropy may be a state of function that’s often erroneously mentioned because of the state of disorder of a system.

Complete step by step answer:
We must have to know that entropy \[\left( S \right)\] is additionally the topic of the second and third law of thermodynamics, which describes the changes in entropy with reference to the system and surroundings, and therefore the entropy of the substances respectively.
Energy can flow from one direction to another form, this is explained by the second law of thermodynamics.
Second law of thermodynamics: sum of the entropy of the system and its surrounding always increases.
Third law of thermodynamics: a perfectly crystalline solid at absolute zero has a zero entropy.
Entropy is measured in $J/K$.
We need to know that the spontaneity of the method appears to be related to a highly ordered system getting to a less-ordered, a uniform state. The \[\Delta S\] of the environment increases enough due to the exothermicity of the reaction that it over compensates for the negative $\Delta S$ of the system.
$\Delta {S_{total}} = \Delta {S_{system}} + \Delta {S_{surroundings}}$
For a spontaneous process, the $\Delta {S_{total}}$ must be positive, i.e., $\Delta {S_{total}} > 0$ .
Total entropy change is always positive is the correct answer.

So, the correct answer is Option D.

Note: We must know that entropy is not a measure if energy itself but of how energy is described within a system. It is enthalpy $(H)$ thermodynamic quantity of the system. We need to know that entropy and enthalpy are somehow related and gives Gibbs free energy $(G)$ of the system, which tells us about the spontaneity of a process.
Change in Gibbs free energy is given by the equation,
$\Delta G = \Delta H - T\Delta S$
Where,
$T$ is temperature
$\Delta H$ is change in enthalpy
$\Delta G$ is change in Gibbs free energy
$\Delta S$ is change in entropy.