Answer:
a) HCN and NaCN
Explanation:
Buffer solution is made by mixing either a weak base and its salt like ammonia and ammonium chloride or weak acid and its salt like HCN and its salt NaCN or acetic acid and sodium acetate .
In the given option , all of the rest of option has either strong acid or strong base or both . In the last option both are salt so it can not form buffer solution .
It is capable of keeping the pH of the solution constant .