The correct answer is B. Formaldehyde and acetaldehyde.
Cannizzaro’s reaction is a chemical reaction between two aldehydes in the presence of a base. The reaction involves the oxidation of one aldehyde to the corresponding carboxylic acid and the reduction of the other aldehyde to the corresponding alcohol.
Formaldehyde and acetaldehyde are both primary aldehydes. Primary aldehydes can undergo Cannizzaro’s reaction because they have a hydrogen atom on the α-carbon atom. This hydrogen atom is abstracted by the base, which creates a carbanion. The carbanion then attacks the carbonyl group of the other aldehyde, resulting in the formation of a tetrahedral intermediate. The tetrahedral intermediate collapses, resulting in the formation of the carboxylic acid and the alcohol.
Benzaldehyde is a secondary aldehyde. Secondary aldehydes cannot undergo Cannizzaro’s reaction because they do not have a hydrogen atom on the α-carbon atom. Therefore, the base cannot abstract a hydrogen atom from the secondary aldehyde, and the reaction cannot proceed.
Here is a diagram of the Cannizzaro reaction between formaldehyde and acetaldehyde:
In this reaction, formaldehyde is oxidized to formic acid and acetaldehyde is reduced to ethanol.