Which of the following complexes exhibits four isomers?

Identify the diamagnetic octahedral complex ions from below $:$
$A. [Mn(CN)_6]^{3-}$
$B. [Co(NH_3)_6]^{3+}$
$C. [Fe(CN)_6]^{4-}$
$D. [Co(H_2O)_3F_3]$
Choose the correct answer from the options given below $:$

$A$ reaction of cobalt $(III)$ chloride and ethylenediamine in a $1:2$ mole ratio generates two isomeric products $A$ (violet coloured) and $B$ (green coloured). $A$ can show optical activity,but $B$ is optically inactive. What type of isomers do $A$ and $B$ represent?

The metal ion whose electronic configuration is not affected by the nature of the ligand and which gives a violet colour in non-luminous flame under hot condition in borax bead test is

What are the complex ions $[Co(NH_3)_5(NO_2)]^{2+}$ and $[Co(NH_3)_5(ONO)]^{2+}$ called?

Which complex is likely to show optical activity?