Step 1: Understand the definition of entropy.
Entropy is a thermodynamic quantity that is often associated with the amount of disorder or randomness in a system. It is a state function that increases as the system becomes more disordered.
Step 2: Explain the relationship between entropy and disorder.
In thermodynamics, the greater the disorder (or randomness) in a system, the higher the entropy. Entropy is also a measure of the unavailability of a system's energy to do work.
Step 3: Conclusion.
Thus, the correct answer is that entropy is a measure of the disorder of the system.
Final Answer: Entropy of a system is a measure of disorder of the system.