Final answer:
Entropy is a measure of the disorder or randomness in a system, with higher entropy indicating more disorder and less available energy to do work. It cannot decrease in a closed system according to the second law of thermodynamics.
Step-by-step explanation:
In information theory, entropy is a measure of the uncertainty associated with a random variable. The correct answer to the student's question is A) Entropy. Entropy is defined as a measure of the degree of randomness or disorder of a system. For instance, gases have higher entropy than liquids, and liquids have higher entropy than solids, showing that the more disordered the state, the higher the entropy. Entropy also indicates the reduced availability of a system's energy to do work.
Furthermore, the entropy of a system reflects how many different ways the system can be arranged at the microscopic level while preserving the same internal energy. Importantly, the second law of thermodynamics tells us that the entropy of a closed system cannot decrease over time, meaning that the tendency is for systems to become more disordered.
Entropy can also be understood in everyday terms: A neatly organized room has low entropy, but as it becomes messier, its entropy increases. This concept is applicable beyond physical systems—it's also used in fields like computer science, for example, to measure the randomness in sequences such as language encoding.