In nucleophilic substitution reaction, order of halogens as incoming (attacking) nucleophile is: $$I^- > Br^- > Cl^-$$. The order of halogens as departing nucleophile should be :

The ease with which a group can leave the substrate in a nucleophilic substitution depends on how stable the group is after departure. Such a group is called a leaving group.

Concept: A good leaving group is the conjugate base of a strong acid. The weaker the basicity of the anion, the more stable it is, and hence the better it can depart.

Among the halide ions, acid strength of the corresponding hydrohalic acids increases down the group:
$$HI \gt HBr \gt HCl$$

Therefore, basicity of their conjugate bases follows the reverse order:
$$I^- \lt Br^- \lt Cl^-$$

Less basic (more stable) ⇒ better leaving group. Hence the leaving-group ability follows the order
$$I^- \gt Br^- \gt Cl^-$$

Thus, the order of halogens as departing nucleophiles is exactly the same as their order as incoming nucleophiles given in the question.

Correct order: $$I^- \gt Br^- \gt Cl^-$$

This matches Option B.