Which of the following is relatively weak acid  $?\left({\mathrm{pK}}_{\mathrm{a}}\right.$ values are given in brackets)

When the compound is weak acid then the value of its ka will be low.

Since, $pKa\propto \frac{1}{Ka}$ 

The compound having the highest amount of $pKa$ will be least acidic.

So, from the given options, HD has the highest $pKa$ value i.e., 9.62. Hence, it will be weak acid.

Therefore, the correct option is (D).