Reaction Combinations Producing Hydrogen Gas

To determine which combination produces hydrogen gas ($\text{H}_2$), let us analyze the chemical reactions involved in each option:

1. Reaction of $\text{Fe}$ metal with concentrated $\text{HNO}_3$ (Option A): Concentrated nitric acid ($\text{HNO}_3$) is a powerful oxidizing agent. When iron ($\text{Fe}$) reacts with concentrated $\text{HNO}_3$, it becomes passive due to the formation of an insoluble, protective oxide film ($\text{Fe}_3\text{O}_4$) on its surface. As a result, no $\text{H}_2$ gas is produced.

2. Reaction of $\text{Cu}$ metal with concentrated $\text{HNO}_3$ (Option B): Copper ($\text{Cu}$) reacts with concentrated nitric acid to yield nitrogen dioxide ($\text{NO}_2$) gas instead of $\text{H}_2$: $\text{Cu}(s) + 4\text{HNO}_3(conc.) \rightarrow \text{Cu(NO}_3)_2(aq) + 2\text{NO}_2(g) + 2\text{H}_2\text{O}(l)$

3. Reaction of $\text{Zn}$ metal with $\text{NaOH}(aq)$ (Option C): Zinc ($\text{Zn}$) is an amphoteric metal. It reacts with strong aqueous bases like sodium hydroxide ($\text{NaOH}$) to form sodium zincate and liberates hydrogen gas ($\text{H}_2$): $\text{Zn}(s) + 2\text{NaOH}(aq) \rightarrow \text{Na}_2\text{ZnO}_2(aq) + \text{H}_2(g)\uparrow$ Therefore, this reaction produces $\text{H}_2$ gas.

4. Reaction of $\text{Au}$ metal with $\text{NaCN}(aq)$ in the presence of air (Option D): Gold ($\text{Au}$) dissolves in a sodium cyanide solution in the presence of atmospheric oxygen (air) to form a soluble coordination complex (MacArthur-Forrest process): $4\text{Au}(s) + 8\text{NaCN}(aq) + \text{O}_2(g) + 2\text{H}_2\text{O}(l) \rightarrow 4\text{Na}[\text{Au(CN)}_2](aq) + 4\text{NaOH}(aq)$ No $\text{H}_2$ gas is evolved in this reaction.

Thus, the combination that produces $\text{H}_2$ gas is Option C.