Which one of the following substituents at para-position is most effective in stabilizing the phenoxide ion?

$$-CH_3$$ (Option A): This is an electron-donating group through the $$+I$$ (inductive) effect and hyperconjugation. It destabilizes the phenoxide ion.

$$-OCH_3$$ (Option B): It can donate into the ring via the $$+M$$ (mesomeric) effect. This makes it a strong electron-donating group that destabilizes the ion.

$$-COCH_3$$ (Option C): This acetyl group is a strong electron-withdrawing group. It exerts both a $$-I$$ effect and a powerful $$-M$$ (resonance withdrawing) effect.

Because it is at the para-position, the negative charge from the phenoxide oxygen can be delocalized through the ring directly onto the oxygen of the carbonyl group. This is the most effective form of stabilization.