Chemistry · Fundamental Principles of Organic Chemistry

Nucleophilicity increases as the density of negative charge increases. An anion is always a better nucleophile than a neutral molecule, so the conjugate base is always a better nucleophile. A highly electronegative atom is a poor nucleophile because it is unwilling to share its electrons. A has ring system so increased charge density thus stronger nucleophile. H-O^- is a strong nucleophile as it has a negative charge and a small size, which allows it to easily donate its lone pair of electrons to form a bond with a positive or electrophilic atom or group.  

C₆H₅O, or phenoxide ion, is a weak nucleophile because the negative charge on the oxygen is delocalized through resonance with the aromatic ring, making the oxygen less nucleophilic. Hence, it is an incorrect option.

H-O^- is a strong nucleophile as it has a negative charge and a small size, which allows it to easily donate its lone pair of electrons to form a bond with a positive or electrophilic atom or group.

NH₃ is a weak nucleophile because it has a lone pair of electrons on the nitrogen atom, but it is not as basic as hydroxide ion or alkoxide ion. Therefore, it is less reactive than hydroxide ion and alkoxide ion as a nucleophile. Hence, it is an incorrect option.

C₂H₅O^- is a weaker nucleophile than H-O- because it is a larger and less electronegative atom than oxygen, so it is less able to donate its electrons to form a bond. Hence, it is an incorrect option.