Solutions of a halogen and a sodium halide are mixed which mixture darkens in colour because a reaction occurs

Chlorine and sodium iodide. (see options attached)

Explanation:

Options:

(A) bromine and sodium chloride (B) bromine and sodium fluoride (C) chlorine and sodium fluoride (D) chlorine and sodium iodide

In the reactivity series, chlorine would displace all the other halogens down the group. The order goes:

The order of reactivity is chlorine > bromine > iodine.

Of all the halogens, iodine is the darkest in colour. Hence, the most likely mixture to give off dark coloration after displacement reaction would be that of chlorine and sodium iodide.

I hope this explanation was easy to understand.

"the periodic table is arranged in order of atomic number."