ANS:A - pressure

Gas turbines typically employ a constant pressure cycle. Explanation: Gas turbines operate on the Brayton cycle, which is a constant pressure cycle. The Brayton cycle consists of four main processes:

1. Isentropic Compression: Air is drawn into the compressor and compressed isentropically (i.e., with no heat transfer) to increase its pressure and temperature.
2. Isobaric Combustion: The compressed air enters the combustion chamber where fuel is injected and burned at constant pressure. This process increases the temperature of the air-fuel mixture.
3. Isentropic Expansion (Power Generation): The high-pressure, high-temperature gases from the combustion chamber expand through the turbine, driving it and generating mechanical power. This expansion occurs isentropically, and the pressure decreases while the volume increases.
4. Isobaric Heat Rejection: After passing through the turbine, the exhaust gases are discharged to the atmosphere at constant pressure, completing the cycle.

In a gas turbine, the combustion process occurs at constant pressure, which distinguishes it as a constant pressure cycle. This is in contrast to other thermodynamic cycles, such as the Otto cycle (used in spark-ignition engines) and the Diesel cycle (used in compression-ignition engines), which involve processes occurring at constant volume and constant temperature, respectively. Therefore, the correct answer is pressure. Gas turbines employ a constant pressure cycle, known as the Brayton cycle, for their operation.