H–H, X–X and H–X bond energies are 104 Kcal/mole 60Kcal/ mole and 101Kcal/mole. Assuming the electro-negativity of hydrogen
to be 2.1 the electronegativity of unkonwn element X is $\text{ (take }\sqrt{19}\text{ as }4.36)$

The mathematical expression for electronegativity can be written as:

${\mathrm{X}}_{\mathrm{A}}-{\mathrm{X}}_{\mathrm{B}}=0.208\sqrt{\mathrm{\Delta }}$

Calculating the value of $∆$ we get:

$\Delta =E_{H-X}-\frac{1}{2}\left[E_{H-H}+E_{X_{-}X}\right]$

$\Rightarrow \mathrm{\Delta }=20$

Substituting the calculated value in the initial equation and solving we get:

$\Rightarrow {\mathrm{X}}_{\mathrm{A}}-{\mathrm{X}}_{\mathrm{B}}=0.208\sqrt{20}$

$\Rightarrow {\mathrm{X}}_{\mathrm{A}}-{\mathrm{X}}_{\mathrm{B}}=0.93$

$\Rightarrow {\mathrm{X}}_{\mathrm{A}}-2.1=0.93$

${\mathrm{X}}_{\mathrm{A}}=0.93+2.1=3$

Therefore, option B) 3 is the correct answer.