Among the properties (a) reducing (b) oxidizing (c) complexing, the set of properties shown by ${\text{CN}}^{-}$ ion towards metal species is:

The cyanide ion (${\text{CN}}^{-}$) is a powerful complexing agent due to its strong field ligand nature and ability to form stable complexes with transition metals. It also acts as a reducing agent in several reactions (e.g., in the extraction of gold/silver or reduction of ${\text{Cu}}^{2+}$). It does not act as an oxidizing agent.