Identify a catalyst used in Stephen reaction from the following.

The Stephen reaction involves the reduction of an alkyl cyanide (nitrile) to the corresponding aldehyde.

The reagents used in this reaction are stannous chloride ($\text{SnCl}_2$) and hydrochloric acid ($\text{HCl}$).

The reaction proceeds via the formation of an imine hydrochloride intermediate, which upon hydrolysis yields the aldehyde.

$R-C \equiv N + \text{SnCl}_2 + \text{HCl} \rightarrow R-CH=NH \cdot \text{HCl} \xrightarrow{H_3O^+} R-CHO + \text{NH}_4^+$

Therefore, the correct catalyst/reagent is $\text{SnCl}_2$ and $\text{HCl}$.

Answer: $\text{SnCl}_2$ and HCl