What is the primary concept of entropy as it relates to the second law of thermodynamics?

• A. Entropy is the measure of energy in a system that is available to do work.
• B. Entropy is the measure of the total energy of a system.
• C. Entropy is the energy required to change a substance from one phase to another.
• D. Entropy is the measure of disorder or randomness in a system.

Answer: (D)