The most reactive compound towards formation of cyanohydrin on treatment with HCN followed by acidification is

Cyanohydrin will be formed easily by those compounds in which electron withdrawing group is present. Electron withdrawing will have –I-Effect which will have more positive charge on the carbonyl carbon atom decreasing the electron density and increasing the tendency of the HCN to attack. So, from the given options in p-nitrobenzaldehyde, nitro group is electron withdrawing group.