Correct option (d) -OCH3
Explanation:
Larger the value of pKa, smaller will be its acidity. Out of the four groups, -COOH, -NO2 and -CN are e- withdrawing which makes benzoic acid more acidic whereas --OCH3 is e donating which reduces the acidity (makes H+ less easily available). pKa value increases if -OCH3 is present at para-position of benzoic acid.