An aromatic carboxylic acid [A] which readily sublimes on heating, produces compound [B] on…

An aromatic carboxylic acid [A] which readily sublimes on heating, produces compound [B] on treatment with $\text{PCl}_5$. Compound [B], when reduced in the presence of $\text{Pd}$ catalyst over $\text{BaSO}_4$ poisoned by sulphur in xylene solution gives compound [C]. When compound [C] is condensed in the presence of alcoholic $\text{KCN}$, it gives compound [D]. (Molecular formula of compound [D] is $\text{C}_{14}\text{H}_{12}\text{O}_2$) Identify the compounds [A], [B], [C] and [D].