Step 1: State the requirement.
A drying agent for ammonia must absorb water without reacting with \( NH_3 \). Ammonia is basic, so acidic desiccants are ruled out.
Step 2: Test P4O10.
\( P_4O_{10} \) is an acidic desiccant. It reacts with basic \( NH_3 \) and cannot be used.
Step 3: Test conc. H2SO4.
Concentrated \( H_2SO_4 \) is a strong acid. It reacts with \( NH_3 \): \[ 2NH_3 + H_2SO_4 \rightarrow (NH_4)_2SO_4 \] So it cannot be used.
Step 4: Test anhydrous CaCl2.
Anhydrous \( CaCl_2 \) forms a complex with ammonia: \( CaCl_2 \cdot 8NH_3 \). This reaction consumes ammonia, so it cannot be used.
Step 5: Test CaO.
Calcium oxide is a basic oxide. It absorbs water: \( CaO + H_2O \rightarrow Ca(OH)_2 \). Being basic, it does NOT react with the basic gas ammonia.
Step 6: State the answer.
CaO is the correct drying agent for ammonia. \[ \boxed{CaO} \]