Catalyst SnCl₂/HCl is used in which of the following method of synthesis of aldehyde? 

Stephen's reduction 

The correct answer is option 1. Stephen's reduction.

1. Stephen's Reduction:

An alkyl or aryl cyanide dissolved in ether is reduced with stannous chloride and \(HCl\) to give aliphatic or aromatic aldehydes. The reaction proceeds by the formation of aldimine hydrochloride (present as stannichloride), which are not stable and hydrolyse to give aldehydes.

2. Cannizzaro Reduction:

The Cannizzaro reduction is  a base-catalyzed disproportionation of aldehydes that have no \(\alpha \)-hydrogens.

We know that converting an aldehyde into an alcohol is a reduction, while going from an aldehyde to a carboxylic acid is an oxidation reaction. So, we can say that half of the aldehyde is oxidized and half of it is reduced. Therefore, in summary, the Cannizzaro reaction is a base-catalyzed disproportionation of aldehydes that have no ɑ-hydrogens.

3. Clemmensen's Reduction:

The name of the chemical reaction is already given as Clemmensen’s reduction. This is named after a Danish chemist who goes by Erik Christian Clemmensen.

It is very popular for its specificity and less complicated procedures that have to be carried out in the laboratory for the formation of products. As it is very clear, the reaction in overall is a reduction process where addition of hydrogen atoms takes place. The reagents only target carbonyl carbons and reduce them to alkane groups.

The general formula of the reaction is as follows:

4. Rosenmund's Reduction:

Acyl chlorides can be reduced into aldehydes with hydrogen in boiling xylene using palladium or platinum as catalyst supported on barium sulphate. This reaction is called Rosenmund’s reduction.

Thus, the correct answer is Stephen's reduction.