Draw different resonance structures of the following:
$(a)$ $CH_3COO^{-}$
$(b)$ $CH_2=CH-CHO$
$(c)$ Aniline $(C_6H_5NH_2)$
$(d)$ $C_6H_5NO_2$
$(e)$ Phenol

(N/A) Resonance structures represent the delocalization of $\pi$-electrons or lone pairs within a molecule.
$(a)$ $CH_3COO^-$: The negative charge is delocalized between the two oxygen atoms,resulting in two equivalent resonance structures.
$(b)$ $CH_2=CH-CHO$: The $\pi$-electrons delocalize towards the electronegative oxygen atom,creating polar resonance structures.
$(c)$ Aniline $(C_6H_5NH_2)$: The lone pair on the nitrogen atom participates in resonance with the benzene ring,increasing electron density at the ortho and para positions.
$(d)$ $C_6H_5NO_2$: The nitro group is electron-withdrawing,pulling $\pi$-electrons from the benzene ring towards the nitrogen atom,creating positive charges at ortho and para positions.
$(e)$ Phenol: The lone pair on the oxygen atom of the $-OH$ group delocalizes into the benzene ring,activating the ring towards electrophilic substitution.