Which of the following elements is the most reactive?
A. At
B. Cl
C. I
D. Br

B.  Cl.

Explanation:

These are Group  7 elements whose reactivity reduces as you go down the group. Chlorine  has a smaller atoms than the rest and has a stronger  tendency to grab an electron in its valency shell. This is because its nucleus is closer to the outer electrons than the other elements.

the number of valence electrons for each molecule or ion is shown beneath the structure. remember hydrogen will not have more than two electrons. this hydrogen is part of a covalent bond (sharing two electrons). a double bond here would cause hydrogen to share four electrons with phosphorus.

i think that the answer is c