The correct answer is option 'b) Reversed Brayton'.

Explanation:
Aircraft refrigeration systems are used to cool the cabin and cargo compartments of the aircraft to provide a comfortable environment for passengers and to preserve perishable goods. These systems typically use a thermodynamic cycle to achieve the desired cooling effect.

The reversed Brayton cycle, also known as the Brayton refrigeration cycle or the Brayton cryogenic cycle, is the thermodynamic cycle commonly used in aircraft refrigeration systems. It is a modification of the Brayton cycle, which is used in gas turbine engines for power generation.

The reversed Brayton cycle consists of four main components:
1. Compressor: The compressor is responsible for compressing the refrigerant gas, increasing its pressure and temperature.
2. Heat exchanger: The compressed refrigerant gas then passes through a heat exchanger, where it rejects heat to the surroundings. This causes the refrigerant to condense into a liquid state.
3. Expansion valve: The liquid refrigerant then passes through an expansion valve, which reduces its pressure and temperature.
4. Evaporator: The low-pressure refrigerant liquid enters the evaporator, where it absorbs heat from the surroundings, such as the cabin or cargo compartments of the aircraft. This heat absorption causes the refrigerant to evaporate into a gas state.

The reversed Brayton cycle operates in a similar manner to the Brayton cycle used in gas turbines, but in reverse. In the Brayton cycle, air is compressed and heated, then expanded to produce work. In the reversed Brayton cycle, the refrigerant gas is compressed, then cooled and expanded to remove heat from the surroundings.

The reversed Brayton cycle is an ideal cycle, assuming no losses or irreversibilities. In reality, there will be some losses due to heat transfer and friction, but the cycle provides a good approximation of the actual refrigeration process.

In conclusion, the thermodynamic cycle used in aircraft refrigeration is the reversed Brayton cycle. This cycle allows for the efficient cooling of the aircraft cabin and cargo compartments by removing heat from the surroundings using a compressed and cooled refrigerant.