According to Pauling,the electronegativity difference between two atoms A and B is directly proportional to (where $\varepsilon$ represents bond enthalpy)

$\begin{array}{1 1}2\varepsilon_{AB}-(\varepsilon_{AA}+\varepsilon_{BB})\\\varepsilon_{AB}^2-\varepsilon_{AA}\varepsilon_{BB}\\\varepsilon_{AB}-\sqrt{\varepsilon_{AA}\varepsilon_{BB}}\\ [\varepsilon_{AB}-\sqrt{\varepsilon_{AA}\varepsilon_{BB}} ] ^{1/2}\end{array}$

Answer : $[\varepsilon_{AB}-\sqrt{\varepsilon_{AA}\varepsilon_{BB}} ] ^{1/2}$