Remember, electronegativity is a measure of how strongly an atom bonds to its electrons i.e. the likelihood of an atom to attract an electron.
If you think about it, the less electron shells the atom has, the stronger the electrons will bond to the atom. Furthermore, the greater the atom's atomic mass, the more the electrons will be attracted to it.
Knowing this, we're looking for the element that is farthest to the left and bottom of the periodic table. So Francium (Fr) will be your answer.
