Entropy is the property of a working substance which increases or decreases as heat is supplied or removed in a reversible manner. It is a measure of the randomness or disorder in a system.

Explanation:
1. Reversible Process:
In order to understand the concept of entropy, it is important to grasp the idea of a reversible process. A reversible process is one in which the system and its surroundings can be restored to their original state by reversing the process. It is an idealized process that does not occur in nature, but it serves as a useful theoretical concept.

2. Heat Transfer and Entropy:
When heat is supplied to a system, the energy is transferred from a higher temperature region to a lower temperature region. This transfer of energy increases the entropy of the system. Conversely, when heat is removed from a system, the energy is transferred from a lower temperature region to a higher temperature region, resulting in a decrease in entropy.

3. Randomness and Disorder:
Entropy is often described as a measure of the randomness or disorder in a system. When heat is added to a system, the molecules of the working substance gain energy and move more randomly, increasing the disorder. This increase in disorder is reflected in the increase in entropy.

4. Internal Energy vs Entropy:
While internal energy (option A) is a measure of the total energy of a system, entropy (option B) is a measure of the disorder or randomness in a system. Internal energy does not necessarily change with the addition or removal of heat, but entropy does.

5. External Energy:
External energy (option C) does not directly relate to the increase or decrease of entropy. It refers to the energy associated with the system's surroundings, such as potential energy or kinetic energy. It is not the property that changes with the addition or removal of heat in a reversible manner.

6. Enthalpy:
Enthalpy (option D) is a measure of the heat content of a system at constant pressure. It is related to the internal energy of a system, but it does not directly relate to the increase or decrease of entropy.

Therefore, the property of a working substance which increases or decreases as heat is supplied or removed in a reversible manner is known as entropy.