${\text{CH}}_3{\text{CO}}_2{\text{C}}_2{\text{H}}_5$ on reaction with sodium ethoxide in ethanol gives $A$, which on heating in the presence of acid gives $B$. Compound $B$ is:

This is a Claisen condensation reaction.Step 1: Two molecules of ethyl acetate (${\text{CH}}_3{\text{CO}}_2{\text{C}}_2{\text{H}}_5$) react with ${\text{NaOC}}_2{\text{H}}_5$ to form ethyl acetoacetate (${\text{CH}}_3{\text{COCH}}_2{\text{COOC}}_2{\text{H}}_5$, compound A).Step 2: Heating compound A in the presence of acid ($H^{+}$) leads to hydrolysis and decarboxylation to form the keto-enol tautomer of a cyclic structure or acetone depending on conditions. Based on the provided solution images, the reaction proceeds to form a specific cyclic structure.