$C_6H_5CH_3\xrightarrow{(i)\ CrO_2Cl_2,\ CS_2;\ (ii)\ H_3O^+}C_6H_5CHO$ (Etard reaction), so P is benzaldehyde. Benzaldehyde is also obtained from benzoyl chloride by hydrogenation over Pd/$BaSO_4$ (Rosenmund reduction): $C_6H_5COCl\xrightarrow{H_2,\ Pd/BaSO_4}C_6H_5CHO$.