Q.Which one of the following is the least electronegative element?

Electronegativity is the relative tendency of an atom, within a covalently bonded molecule, to attract the shared electron pair toward itself. Unlike atomic radius or ionisation energy, it is not a property of an isolated atom and cannot be measured directly -- it must be assigned on a scale. On the widely used Pauling scale, H is fixed at 2.1 and F at 4.0, and other values are derived from bond dissociation energies via $(\chi_A - \chi_B) = 0.182\sqrt{E_{AB} - \sqrt{E_{AA}\times E_{BB}}}$; because this is built from bond energies, an element's electronegativity is not a true fixed constant -- it depends somewhat on its bonding partner. …