Answer:
I^- or Cl^-
Step-by-step explanation:
A nucleophile is any reagent that donates an unshared pair of electrons to form a new covalent bond. Nucleophiles are mostly bases also. Nucleophilicity is a kinetic property, it refers to the rate at which a nucleophile replaces a leaving group from an alkyl halide.
Looking at the options, Cl^- and I^- are the best nucleophiles among the options provided in the question.