$−I$ groups ; if more in number and more close to $⎯COOH$ group, more is the value of $K_a$ while $+ I$ group if are more in number and close to $⎯COOH$ group, decreases the value of $K_a$.
$pK_{a}\propto \frac{1}{K_{a}}$
where, $K_a =$ acidic ionisation constant.
(a) It has one $−(I)$ group (i.e. $Cl$) at $C − 2$ position.
(b) It has also one $(−)I$ group (i.e. $Cl$) at $C− 3$ position.
(c) It has three $(−)I$ groups (i.e. $Cl$) at $C − 2$ position.
(d) It has one $(−)I$ group (i.e. $Cl$) at $C − 2$ position.
Thus, option $CCl_3COOH$ is the correct answer.
