Explanation:


Irreversibility is the process in which the entropy of a system increases. The main cause for the irreversibility is the combination of various factors. The detailed explanation is given below.

Mechanical and Fluid Friction:


Mechanical and Fluid friction are the two main causes of irreversibility. In a mechanical system, friction is caused by the contact between two surfaces. This contact results in the generation of heat energy. Fluid friction, on the other hand, is caused by the resistance offered by fluids to the motion of objects. This resistance also results in the generation of heat energy. The heat generated due to friction cannot be completely converted into work, which leads to an increase in the entropy of the system.

Unrestricted Expansion:


Unrestricted expansion is another cause of irreversibility. When a gas expands freely, it does not perform any work. The energy of the gas is converted into heat, which increases the entropy of the system. This process cannot be reversed, as it is not possible to convert heat energy completely back into work.

Heat Transfer with a Finite Temperature Difference:


Heat transfer with a finite temperature difference is also a cause of irreversibility. When two systems with different temperatures come into contact with each other, heat flows from the hotter system to the colder system. This process results in an increase in the entropy of the system. This increase in entropy cannot be reversed, as it is not possible to completely convert heat energy back into work.

Conclusion:


In conclusion, the main cause for irreversibility is the combination of various factors like mechanical and fluid friction, unrestricted expansion, and heat transfer with a finite temperature difference. These factors lead to an increase in the entropy of the system, which cannot be reversed completely. Therefore, the correct answer is option 'D'.