b
Since the leaving group breaks away as a base, it is easier to displace weaker bases as compared to stronger bases. Thus less basic the substituent. the more easily it is displaced. Since the basic strength of the given groups is in order. \({I^ - } < B{r^ - } < Cl^-\) Thus the order of halogen leaving groups is  \({I^ - } > B{r^ - } > C{l^{^ - }}\)