In which of the following cases will a violet coloration be observed ?
A. An alkaline solution sodium nitroprusside is treated with a solution of `Na_(2)S`.
B. A solution of sodium cobaltinitrite is treated with one of KCl.
C. A solution of `Mn(N_(3))_(2)` is treated with sodium bismuthate or red lead in presence of concentrated `HNO_(3)`
D. A solution of sodium nitroprusside in aqueous NaOH is treated with `Na_(2)SO_(2)O_(3)`.
