What happens when benzaldehyde undergoes nucleophilic addition with HCN followed by reaction with water and HCl/Δ?  

Mandelic acid is produced 

The correct answer is option 1. Mandelic acid is produced.

When benzaldehyde undergoes nucleophilic addition with hydrogen cyanide \((HCN)\), followed by reaction with water and \(HCl\) (with heating), the product formed is Mandelic acid.

First, benzaldehyde reacts with hydrogen cyanide \((HCN)\) in a nucleophilic addition reaction, forming mandelonitrile. Mandelonitrile, upon treatment with water and hydrochloric acid \((HCl)\) and heating, undergoes hydrolysis. The nitrile group \((-CN)\) is hydrolyzed to a carboxylic acid group \((-COOH)\), resulting in mandelic acid:

Thus, the final product is: Mandelic acid.