Which of the following gas samples contains the same number of...

Molar mass of $\mathrm{NH}_3 = 14 + 3 = 17\text{ g/mol}$

Number of moles of $\mathrm{NH}_3 = \dfrac{3.4}{17} = 0.2\text{ mol}$

Number of molecules is directly proportional to the number of moles. We check the number of moles for each option:

For $\mathrm{H}_2\mathrm{S}$, molar mass $= 34\text{ g/mol}$. Moles $= \dfrac{3.4}{34} = 0.1\text{ mol}$

For $\mathrm{O}_2$, molar mass $= 32\text{ g/mol}$. Moles $= \dfrac{8.0}{32} = 0.25\text{ mol}$

For $\mathrm{SO}_2$, molar mass $= 64\text{ g/mol}$. Moles $= \dfrac{12.8}{64} = 0.2\text{ mol}$

For $\mathrm{CO}_2$, molar mass $= 44\text{ g/mol}$. Moles $= \dfrac{4.4}{44} = 0.1\text{ mol}$

Since $12.8\text{ g}$ of $\mathrm{SO}_2$ has $0.2\text{ mol}$, it contains the same number of molecules as $3.4\text{ g}$ of $\mathrm{NH}_3$.

Answer: $12.8\text{ g}$ of $\mathrm{SO}_2$