Answer: A. Ammonia is alkaline and less dense than air (\(M_r = 17\) vs air \(\approx 29\)). Two requirements: a drying agent that does not react with \(\text{NH}_3\), and a collection method that exploits the lower density.
Calcium oxide (CaO) is a basic drying agent — fine for the alkaline gas \(\text{NH}_3\). Concentrated sulfuric acid is acidic and would react with \(\text{NH}_3\): \(\text{H}_2\text{SO}_4 + 2\text{NH}_3 \rightarrow (\text{NH}_4)_2\text{SO}_4\). So options C and D are wrong on the drying agent.
Since \(\text{NH}_3\) is less dense than air, it rises and is collected by upward delivery (gas enters at the bottom of an inverted jar). Option B uses the right drying agent but wrong collection direction. Only A combines both correctly.
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