Answer
(a)
Work Step by Step
Every good buffer needs:
1. A weak acid.
(a): $CH_3CO_2H$;
(b): Missing;
(c): $CH_3CO_2H$ and $HCO_2H$;
(d): Missing;
2. The conjugate base of the weak acid (or a salt that contains it):
(a): $KCH_3CO_2H$;
(c): Missing;
3. They have to be in similar concentrations:
(a) $0.20M$ $KCH_3CO_2$ and $0.20M$ $CH_3CO_2H$;
Therefore, the correct answer is (a);