@ K_a increases in benzoic acid when substituent " x " is bonded at para-position, then " x " is

Step-by-step solution

Larger the value of p K_ a r smaller will be its acidity. Out of the four groups, - COOH ,- NO ₂ and - CN are e⁻ withdrawing which makes benzoic acid more acidic whereas - OCH ₃ is e⁻ donating which reduces the acidity (makes H ⁺ less easily available). p K_a value increases if OCH ₃ is present at para-position of benzoic acid.