The strongest nucleophile towards $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{Br}$ in methanol among the following anions is

In a protic solvent, nucleophilicity increases down a group because the larger atom is more polarisable and less strongly hydrogen bonded. Thiolate is therefore much better than acetate, chloride and fluoride, even though it is a weaker base than some of them (compare fluoride, a stronger base but tightly solvated).