Regenerative steam cycle is a modification of the Rankine cycle, which is commonly used in power plants to generate electricity. In a regenerative steam cycle, the feedwater is heated by extracting heat from the steam at various stages of the cycle before it enters the boiler. This process of heat extraction is known as regeneration.

Increased work output per unit mass of steam:
- In a regenerative steam cycle, the heat extracted from the steam during regeneration is used to preheat the feedwater before it enters the boiler.
- This preheating of the feedwater increases its temperature, resulting in an increase in the average temperature at which heat is added in the boiler.
- According to the Carnot principle, the maximum work output of a heat engine is directly proportional to the temperature difference between the heat source and the heat sink.
- By increasing the average temperature at which heat is added in the boiler, the regenerative steam cycle increases the temperature difference, leading to increased work output per unit mass of steam.

Increased thermal efficiency:
- The thermal efficiency of a power cycle is defined as the ratio of the net work output to the heat input.
- In a regenerative steam cycle, the heat extracted from the steam during regeneration is utilized to preheat the feedwater, reducing the amount of heat that needs to be supplied in the boiler.
- As a result, the heat input to the cycle is reduced, while the work output remains the same (or increases due to the increased temperature difference).
- This reduction in heat input leads to an increase in the thermal efficiency of the regenerative steam cycle.

Decreased work output per unit mass of steam as well as increased thermal efficiency:
- It is important to note that the regenerative steam cycle does not increase both work output per unit mass of steam and thermal efficiency simultaneously.
- The increase in work output per unit mass of steam is achieved by increasing the temperature difference, while the increase in thermal efficiency is achieved by reducing the heat input.
- These two objectives are often contradictory, as increasing the temperature difference requires more heat input.
- Therefore, it is not possible to simultaneously increase both work output per unit mass of steam and thermal efficiency in a regenerative steam cycle.
- The correct answer, option D, reflects this contradiction, stating that the work output per unit mass of steam decreases while the thermal efficiency increases.