Ethanal is warmed with dilute $\mathrm{NaOH}$ to give A, and A is heated to give B by loss of water. B is

Aldol addition of two ethanal molecules gives $\mathrm{CH}{}_3\mathrm{CH}(\mathrm{OH})\mathrm{CH}{}_2\mathrm{CHO}$ (3-hydroxybutanal, A). On heating, dehydration through the alpha hydrogen gives the conjugated $\mathrm{CH}{}_3\mathrm{CH}=\mathrm{CHCHO}$ (but-2-enal, B).