Entropy of a thermodynamic system does not change when this system is used for?
A. Conduction of heat from a hot reservoir to a cold reservoir
B. Conversion of heat into work isobarically
C. Conversion of heat into internal energy isochorically
D. Conversion of work into heat isochorically

Hint: This problem is based on the entropy and we know that in a particular thermodynamic system, entropy varies with mass, temperature, heat exchange, etc. Hence, analyze each option given with the scientific approach and check for which option entropy of the system remains constant i.e., neither increase nor decrease.

Complete answer:
We know that Entropy represents the ‘degree of randomness’ of a thermodynamic system and directly varies with it.
If the degree of randomness of a system is high, the entropy of a system will automatically high.

Let us consider all the four options given and check whether the Entropy is constant or not for a particular given process.
In Case 1. When there is a conduction of heat from a hot reservoir to a cold reservoir, the degree of randomness will decrease resulting in a decrease in entropy.
In Case 2. When there is a conversion of heat into work isobarically (means at constant pressure), the degree of randomness will decrease due to an increase in volume resulting in a decrease in entropy.
In Case 3. When there is a conversion of heat into internal energy isochorically (means at constant volume), the degree of randomness will increase due to an increase in internal energy of the system resulting in an increase in entropy.
In Case 4. When there is a conversion of work into heat isochorically (means at constant volume), the degree of randomness will remain the same resulting in entropy being constant.

Thus, the Entropy of a thermodynamic system does not change when the system is used for conversion of work into heat isochorically.

Hence, the correct option is (D) Conversion of work into heat isochorically.

Note: Since this is a theoretical based conceptual problem on entropy of a thermodynamic system and we know that entropy changes, during different conversions like- heat into internal energy or heat into work and vice-versa, hence, it is advised to properly analyze the process for which the conversion occurs before giving a final answer to the given problem.