The correct answer is option 'C' - Stephen reaction.

The Stephen reaction is a method used to prepare aldehydes from nitriles. It involves the reduction of nitriles to imines using stannous chloride (SnCl2) in the presence of hydrochloric acid (HCl). The imines obtained from this reaction can be further hydrolyzed to yield aldehydes.

Here is a detailed explanation of the Stephen reaction:

1. Reaction Equation:
The reaction can be represented by the following equation:
RCN + SnCl2 + HCl → RCH=NCl + SnCl4

2. Role of Stannous Chloride:
Stannous chloride (SnCl2) acts as a reducing agent in the reaction. It converts the nitrile group (RCN) into an imine group (RCH=NCl). The reduction involves the transfer of a hydride ion (H-) from SnCl2 to the nitrile carbon, resulting in the formation of the imine.

3. Role of Hydrochloric Acid:
Hydrochloric acid (HCl) serves as a catalyst in the reaction. It helps in the activation of the nitrile group by protonating it, making it more susceptible to reduction. The acid also helps in maintaining the acidic conditions required for the reaction to proceed.

4. Formation of Imines:
In the presence of SnCl2 and HCl, the nitrile group undergoes nucleophilic addition with SnCl2. This leads to the formation of an intermediate complex, followed by the elimination of a chloride ion. The resulting imine (RCH=NCl) is stabilized by resonance, where the nitrogen atom donates its lone pair of electrons to the carbon-nitrogen double bond.

5. Hydrolysis to Aldehydes:
The imines obtained from the Stephen reaction can be hydrolyzed to yield aldehydes. This is achieved by treating the imine with a suitable reagent, such as dilute acid or base. The hydrolysis reaction involves the cleavage of the carbon-nitrogen double bond, resulting in the formation of an aldehyde.

In summary, the Stephen reaction is a method used to prepare aldehydes from nitriles. It involves the reduction of nitriles to imines using stannous chloride in the presence of hydrochloric acid. The imines obtained can be further hydrolyzed to yield aldehydes.