d
Weak acid \(\left( CH _{3} COOH \right)\) and salt of weak acid-strong base \(\left( CH _{3} COONa \right)\) form an acidic buffer.

Sodium acetate \(\left( CH _{3} COONa \right)=0.10\, M\);

Acetic acid \(\left( CH _{3} COOH \right)=0.01\, M\);

\(pH\) of acidic buffer solution is given by

\(pH = pK _{ a }+\log \frac{[\text { Salt }]}{[\text { Acid }]}\)

\(=4.57+\log \left(\frac{0.1}{0.01}\right)\)

\(=5.57\)