I : O2NCH2CH2O–
II : CH3CH2O–
In order to find the more stable one of the two given ions we need to understand that any ion, whether positively charged or negatively charged, is stable if it is somehow able to diminish the charge appearing on it or, is able to delocalise (spread) the charge on more than one atom. Such a delocalisation of charge is possible in I but not in II. Hence, I is more stable. This dispersal/delocalisation of negative charge happens in I due to the presence of the –NO2 group which is electron withdrawing in nature. Due to this electron withdrawal, the charge on the oxygen atom disperses (spreads) and the ion becomes more stable.