$\text{Mode of the grouped data can be found in the following formula:}$
$\text{Mode=l+($\frac{f_1-f_0}{2f_1-f_0-f_2})*h$}$
where
$\text{l= lower limit of modal class}$
$\text{h=Size of class}$
$\text{$f_0$=Frequency of the class preceding the modal class}$
$\text{$f_1$=Frequency of the modal class}$
$\text{$f_2$=Frequency of the class succeeding the modal class}$
Here The maximum class frequency is $35$ and the class interval corresponding to this frequency is $18-24$. Thus, the modal class is $18-24$
$\therefore$ $\text{Mode=18+$(\frac{35-25}{2*35-25-18})*6$}$.
$\text{Mode=18+$(\frac{10}{70-25-18})*6$}$
$\text{Mode=18+$\frac{60}{27}$=20.22}$
Option $A$ is correct here.