Increasing the number of reheating stages in a gas turbine to infinity makes the expansion tend to an isothermal process.

Explanation:

Reheating stages in a gas turbine are used to increase the efficiency of the turbine by reducing the temperature drop during expansion. When the gas expands through the turbine, its temperature decreases. Reheating stages are introduced to increase the temperature of the gas before it expands further. This helps to maintain a higher average temperature during the expansion process, which in turn increases the efficiency of the turbine.

To understand why increasing the number of reheating stages results in an isothermal expansion, let's consider the ideal gas turbine cycle.

1. Ideal Gas Turbine Cycle:
The ideal gas turbine cycle consists of four processes: compression, combustion, expansion, and exhaust. These processes occur in the following sequence:

a) Compression: The air is compressed in a compressor, resulting in an increase in pressure and temperature.
b) Combustion: Fuel is injected into the compressed air, and combustion takes place, increasing the temperature further.
c) Expansion: The high-temperature and high-pressure gas expands through the turbine, doing work on the turbine to produce mechanical power.
d) Exhaust: The exhaust gases are discharged to the atmosphere.

2. Reheating Stages:
Reheating stages are introduced between the expansion process. After the gas expands partially through the turbine, it is reheated to a higher temperature before expanding further. This process is repeated multiple times, with each reheating stage increasing the gas temperature.

3. Effect on Expansion Process:
When the number of reheating stages is increased to infinity, the temperature of the gas remains constant during the expansion process. This is because each reheating stage brings the gas back to the same temperature it had before expansion. As a result, the expansion process tends towards an isothermal process.

4. Isothermal Expansion:
In an isothermal expansion, the temperature of the gas remains constant throughout the process. This is achieved by providing heat to the gas as it expands. In the case of increasing the number of reheating stages to infinity, the gas is continuously reheated to the same temperature, resulting in an isothermal expansion.

Therefore, the correct answer is option 'B': Isothermal.