Answer
The student should choose the $HC$, because it has the closest $pK_a$ to $8.60$.
Work Step by Step
1. Calculate the pKa value for $HA$.
$pKa = -log(Ka)$
$pKa = -log( 2.7 \times 10^{- 3})$
$pKa (HA) = 2.569$
2. Calculate the pKa value for $HB$
$pKa = -log(Ka)$
$pKa = -log( 4.4 \times 10^{- 6})$
$pKa (HB)= 5.357$
3. Calculate the pKa value for $HC$:
$pKa = -log(Ka)$
$pKa = -log( 2.6 \times 10^{- 9})$
$pKa(HC) = 8.585$
Therefore, $HC$ is the better choice for $pH = 8.60$, because it has the closest $pK_a$.
