The structure of A in the given reaction is ​​​​​​​

Alpha hydrogen in aldehydes are slightly acidic and in presence of alkali they release ${\mathrm{H}}^{+}$ions and carbanion.

Ethyl bromide undergoes nucleophilic substitution reaction with the carbanion generated, where bromide nucleophile is replaced by the carbanion to form ketone by ${\mathrm{S}}_{\mathrm{N}}2$ mechansim