The electronegativity of hydrogen is 2.1 and that of chlorine is 3.2. The percent ionic character of the bond in $\mathrm{HCl}$ molecule is approximately

Electronegativity is the tendency of an atom to attract the shared pair of electrons toward itself and ionic compounds are formed when two atoms have a difference in their electronegativities.

The formula to determine the percent ionic character is - $16({\mathrm{x}}_{\mathrm{B}}-{\mathrm{x}}_{\mathrm{A}})+3.5{({\mathrm{x}}_{\mathrm{B}}-{\mathrm{x}}_{\mathrm{A}})}^2$

Where X_A is the electronegativity of atom A and X_B is the electronegativity of atom B

For HCl, X_H = 2.1 and X_Cl = 3.2.

Thus, the percent ionic character of HCl is calculated as-

$\begin{array}{rcl} & & \% \mathrm{ionic} \mathrm{character} \mathrm{in} \mathrm{HCl}\\ & =& 16(3.2-2.1)+3.5{(3.2-2.1)}^2 \\ & =& 16\times 1.1+3.5\times {(1.1)}^2 \\ & =& 17.6+3.5\times 1.21\\ & =& 17.6+4.235\\ & =& 21.835\%\end{array}$

The percent ionic character in HCl is 21.835% which is approximately 20%.

Hence. the correct option is D.