$[Co (en) X_{2} (NO X_{2}) Cl] X^{+}$ contains the ambidentate $NO X_{2} X^{-}$ (bound through N or O). Counting geometrical, optical and linkage isomers together (octahedral, en symmetrical bidentate), the total number of isomers is

Answer: C

M (AA) X_{2} bc

has trans (1) and cis (2 enantiomers) forms: 3 stereoisomers. Each can have

NO X_{2} X^{-}

as nitro or nitrito, which doubles the count. Total

= 3 \times 2 = 6