The aldehydes which will not form Grignard product with one equivalent of Grignard reagents are:

• Grignard reagents are exceptionally strong bases as well as powerful nucleophiles.
• Faced with a choice, an acid-base proton transfer always occurs much faster than a nucleophilic addition to a carbonyl group.
• If a substrate contains an acidic proton ($$-\text{OH}$$, $$-\text{COOH}$$, $$-\text{NH}_2$$), the single equivalent of the Grignard reagent is immediately quenched (destroyed) to form an alkane ($$\text{R--H}$$), leaving no reagent left to attack the aldehyde group.

Evaluating the Compounds:

• (A) Benzaldehyde: Contains no acidic protons. It reacts smoothly to form the Grignard addition product.
• (B) 4-Formylbenzoic acid: Contains a highly acidic carboxylic acid group ($$-\text{CO}_2\text{H}$$). It quenches the reagent instantly. (No addition product)
• (C) 4-Methoxybenzaldehyde: The methoxy group ($$-\text{OCH}_3$$) is an ether and carries no acidic protons. It forms the addition product smoothly.
• (D) 4-(Hydroxymethyl)benzaldehyde: Contains an acidic alcohol group ($$-\text{CH}_2\text{OH}$$). It quenches the reagent instantly. (No addition product)

Conclusion:

Substrates (B) and (D) contain acidic functional groups that destroy the single equivalent of the Grignard reagent via an acid-base reaction, preventing nucleophilic addition to the aldehyde.

Answer: Option D — (B), (D)