Answer
$C_6H_5CO_2H$ and $C_6H_5C{O_2}^-$ are the best pair, because the $pK_a$ for benzoic acid is the closest to 4.50.
Work Step by Step
1. Calculate the pKa value for $HS{O_4}^-$
$pKa = -log(Ka)$
$pKa = -log( 0.012)$
$pKa = 1.92$
2. Calculate the pKa value for $HOCl$
$pKa = -log(Ka)$
$pKa = -log( 3.5 \times 10^{- 8})$
$pKa = 7.46$
3. Calculate the pKa value for $C_6H_5CO_2H$
$pKa = -log(Ka)$
$pKa = -log( 6.5 \times 10^{- 5})$
$pKa = 4.19$
