Explanation:
Firstly, we need to define electronegativity
Electronegativity is defined as the tendency of an atom in a covalent bond to attract the bonding electrons.
The electronegativity of the halogen decreases down the group due to an increase in the radius of the atom resulting in the addition of outermost shells to the shell of the atom. As the number of shells increases, the force attraction between the nucleus and the electron moving around the shell decreases.