Which of the following is correct for acid buffer ? [ salt = S, acid = A ] :

Question

( A ) $pK_a = pH - \frac{[A]}{[S]}$
( B ) $pH = pK_a + \log \frac{[S]}{[A]}$
( C ) $pH = pK_a + \log \frac{[A]}{[S]}$
( D ) $pK_a = pH + \log \frac{[S]}{[A]}$