Which of the following complexes will exhibit optical isomerism?

In nitroprusside ion,the iron and $NO$ exist as $Fe^{II}$ and $NO^+$ rather than $Fe^{III}$ and $NO$. These forms can be differentiated by

The number of optical isomers possible for $[Cr(C_{2}O_{4})_{3}]^{3-}$ is $.....$ .

The spin-only magnetic moment of an octahedral complex of $Fe^{2+}$ in the presence of a strong field ligand in $B.M.$ is $.....$

Which of the following will have three stereoisomeric forms?
$(i) \ [Cr(NO_3)_3(NH_3)_3]$
$(ii) \ K_3[Co(C_2O_4)_3]$
$(iii) \ K_3[CoCl_2(C_2O_4)_2]$
$(iv) \ [CoBrCl(en)_2]$

$[Co(NH_3)_4Cl_2]NO_2$ and $[Co(NH_3)_4Cl.NO_2]Cl$ are $........$ isomers.