# For $(i) I^-, (ii) Cl^{-}, (iii) Br^{-}$. The increasing order of nuclephilicity would be :

$(a)\;Cl^{-} < Br^- < I^- \\ (b)\;I^{-} < Cl^- < Br^- \\(c)\; Br^{-} < Cl^- < I^- \\ (d)\;I^{-} < Br^- < Cl^-$

In case of different nucleophiles, but present in the same group in the periodic table, then the larger is the atomic mass, higher is nucleophilicity of halide ions is
$I ^- > Br ^- > Cl^-$
Hence a is the correct answer.