Identify the reaction being asked

The reaction of a compound with concentrated alkali such as $50\%$ sodium hydroxide giving the corresponding alcohol and acid is the Cannizzaro reaction.

General form: $2RCHO + \text{conc. NaOH} \rightarrow RCH_2OH + RCOONa$ On acidification, the salt gives the corresponding acid.

Condition for Cannizzaro reaction

Cannizzaro reaction is shown by aldehydes that do not contain an $\alpha$-hydrogen.

Check each option

A: Phenol

• Phenol is not an aldehyde.
• It does not undergo Cannizzaro reaction.
• Incorrect.

B: Benzoic acid

• Benzoic acid is already a carboxylic acid.
• It will not give alcohol and acid with $50\%$ NaOH.
• Incorrect.

C: Butanal

• Butanal is an aldehyde, but it has $\alpha$-hydrogen atoms.
• Aldehydes with $\alpha$-hydrogen undergo aldol reaction in presence of base, not Cannizzaro reaction.
• Incorrect.

D: Benzaldehyde

• Benzaldehyde is an aldehyde with no $\alpha$-hydrogen.
• Therefore it undergoes Cannizzaro reaction with concentrated NaOH: $2C_6H_5CHO + \text{NaOH} \rightarrow C_6H_5CH_2OH + C_6H_5COONa$
• On acidification, sodium benzoate gives benzoic acid.
• So it gives the corresponding alcohol and acid.
• Correct.

Final answer

The compound is benzaldehyde.