Which gas can be collected by downward displacement of air?

Air is primarily composed of nitrogen ($\text{N}_2$) and oxygen ($\text{O}_2$).
Molar mass of $\text{N}_2 = 2 \times 14.01 = 28.02 \text{ g/mol}$
Molar mass of $\text{O}_2 = 2 \times 16.00 = 32.00 \text{ g/mol}$
The average molar mass of air is approximately $29 \text{ g/mol}$.

• A. Chlorine ($\text{Cl}_2$): Molar mass of $\text{Cl}_2 = 2 \times 35.45 = 70.90 \text{ g/mol}$
• B. Sulphur(IV) oxide ($\text{SO}_2$): Molar mass of $\text{SO}_2 = 32.07 + (2 \times 16.00) = 32.07 + 32.00 = 64.07 \text{ g/mol}$
• C. Ammonia ($\text{NH}_3$): Molar mass of $\text{NH}_3 = 14.01 + (3 \times 1.01) = 14.01 + 3.03 = 17.04 \text{ g/mol}$
• D. Carbon(IV) oxide ($\text{CO}_2$): Molar mass of $\text{CO}_2 = 12.01 + (2 \times 16.00) = 12.01 + 32.00 = 44.01 \text{ g/mol}$

• Chlorine ($70.90 \text{ g/mol}$) is denser than air.
• Sulphur(IV) oxide ($64.07 \text{ g/mol}$) is denser than air.
• Ammonia ($17.04 \text{ g/mol}$) is less dense than air.
• Carbon(IV) oxide ($44.01 \text{ g/mol}$) is denser than air.

Gases less dense than air are collected by downward displacement of air (where the collection vessel is inverted, and the gas fills it from the bottom up, pushing air out from the bottom opening).

Therefore, ammonia is the gas that can best be collected by downward displacement of air.

The final answer is $\boxed{\text{C}}$.