The correct option is C (a) and (b) above.
A solution of the salt of weak acid and weak base in water is called salt buffer.
a) Ammonium acetate (CH3COONH4) solution is a salt of weak acid (CH3COOH) and weak base (NH4OH) in water.
b) Ammonium cyanide (NH4CN) solution is a salt of weak acid (HCN) and weak base (NH4OH) in water.
Hence both (a) and (b) option are correct.