How many geometrical isomers are theoretically possible for the complex $[Pt(py)(NH_3)(NO_2)Cl BrI]$?

$Cis-trans$ isomerism is exhibited by

The electronic configurations of $Cr^{2+}, Mn^{2+}, Fe^{2+},$ and $Co^{2+}$ are $d^4, d^5, d^6,$ and $d^7$ respectively. Which of the following has the lowest magnetic moment? $[Cr = 24, Mn = 25, Fe = 26, Co = 27]$

$A$ 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 spin-only magnetic moment of $Fe^{2+}$ ion (in $BM$) is approximately

Which of the following does not show optical isomerism?
$(en = \text{ethylenediamine})$