Multi-Step Reaction

Correct Answer: D

First, ethylene glycol reacts with the aldehyde under acidic conditions to yield an acetal, which masks the aldehyde for the Grignard reaction. Step 2 involves reaction of magnesium metal with the aryl bromide to form a Grignard reagent that subsequently reacts with dry ice (CO2) to give a carboxylate. After quenching with aq. acid, the carboxylate is turned into the carboxylic acid and the acetal is hydrolyzed back to the aldehyde.