Final answer:
The best nucleophile for an SN₂ reaction is a strong nucleophile, which is more effective in attacking the electrophilic carbon and displacing the leaving group efficiently, particularly in polar aprotic solvents.
Step-by-step explanation:
For an SN₂ reaction, the best nucleophile is 3) A strong nucleophile. This is because the SN₂ mechanism involves a bimolecular process where both the nucleophile and substrate are involved in the rate-determining step. A strong nucleophile is more likely to successfully attack the electrophilic carbon and displace the leaving group on the substrate.
In contrast to basicity, which is largely related to the equilibria of proton transfer reactions, nucleophilicity is a kinetic property signifying a molecule's ability to donate a pair of electrons to an electron-deficient site, such as carbon. Factors influencing nucleophilicity include charge, solvent, steric hindrance, and the nature of the substituents on the nucleophile. Particularly, polar aprotic solvents are suitable for SN₂ reactions as they enhance the reactivity of nucleophiles.