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In an absorption refrigeration cycle, ammonia is commonly used as the refrigerant due to its effective thermodynamic properties, allowing it to absorb heat efficiently. The correct answer is based on the combination of ammonia and hydrogen, which is utilized as an absorbent in the cycle.

Ammonia vapor is absorbed back into the liquid ammonium solution, allowing the cycle to continue efficiently. The presence of hydrogen can enhance the system's performance by improving the refrigerant's ability to dissipate heat. This characteristic maximizes the cooling effect, making it a preferred choice in specific applications of absorption refrigeration.

The other combinations do not effectively support the absorption refrigeration cycle in the same manner. While ammonia is consistently used as a refrigerant, the absence of hydrogen in their respective pairs reduces the system's overall efficiency, as these combinations do not provide the ideal thermodynamic properties that the ammonia-hydrogen pairing does.