Transition and Inner Transition Elements — Chemistry STD 12 — Question

The hydrogen ion concentration of a buffer solution consisting of a weak acid and its salts is given by
$
\left[ H ^{+}\right]= K _{ a } \frac{[ acid ]}{[\text { salt }]}
$
$
\left.\left[ H ^{+}\right]= K _{ a } \text { [salt }\right]
$
$
\left[ H ^{+}\right]= K _{ a } \text { [acid] }
$
$
\left[ H ^{+}\right]= K _{ a } \frac{[\text { salt }]}{[\text { acid }]}
$