The major product formed in the following reaction is

Concept:

Stetter reaction: The aldehyde is converted from an electrophile to a nucleophile under the reaction conditions. This is accomplished by activation from some catalyst - either cyanide (CN⁻) or thiazolium salt.

→ The active catalyst can be described as the combination of two contributing resonance forms - an ylide or a carbene, both of which portray the nucleophilic character at carbon.

→ The thiazolium ylide or CN⁻ can then add into the aldehyde substrate, forming a cyanohydrin in the case of CN⁻ or the Breslow intermediate in the case of thiazolium salt.

Explanation:

Conclusion:
The correct answer is option 3.