Which of the following are used in an absorption refrigeration cycle?

In an absorption refrigeration cycle, ammonia is commonly used as the refrigerant due to its excellent thermodynamic properties and ability to absorb heat. The correct choice includes a secondary working fluid that assists in the refrigeration process. In the case of ammonia and hydrogen, hydrogen serves as an absorbent where ammonia is evaporated, then absorbed, and subsequently condensed to repeat the cycle. This pairing allows the refrigeration system to function efficiently without relying on mechanical compressors.

The other combinations listed do not include hydrogen as a secondary fluid, which is critical for the absorption process in this context. For instance, while propane and butane can serve as refrigerants, they are typically not used in absorption refrigeration systems with ammonia. Additionally, helium, while a gas that can be involved in some cooling processes, is not compatible with ammonia in the specific role described in the absorption cycle. Therefore, ammonia with hydrogen is the appropriate choice for this refrigeration method, highlighting its unique function in thermodynamic cycles.