Concept:A desiccant must absorb water vapour without reacting with the gas being dried. Since ammonia is basic, the desiccant must not be acidic or form an ammonia complex.
Explanation:Phosphorus(V) oxide and concentrated tetraoxosulphate(VI) acid are acidic drying agents.
Both will react with basic ammonia gas, so they cannot be used to dry it.
Fused calcium chloride also reacts with ammonia to form a complex compound, making it unsuitable.
Calcium oxide removes water vapour by reacting with it:
CaO+H2O→Ca(OH)2.
Because calcium oxide is basic, it does not react with ammonia gas.
Therefore, it can safely dry ammonia without removing or contaminating the gas.
Answer:C. calcium oxide