Which among the following is least electro negative?

Electronegativity of an element depends upon
(a) Size of an atom
(b) The magnitude of a nuclear charge
On going from Al to Ga, The electrons start filling into the d-subshells. Since these intervening electrons do not screen the nuclear charge.
Hence, the electrons in Ga experience more force of attraction than in Al causing a decrease in atomic size and increase in electronegativity.
Thus, Electronegativity decreases from B to Al and then increases due to discrepancies of an atomic size of the elements.