Which of the following forms maximum number of isomers ?

$\begin{array}{l}\left(a\right){\left[\mathrm{Co}(\mathrm{en}{)}_2{\mathrm{Cl}}_2\right]}^{+}\approx {\left[\mathrm{M}(\mathrm{AA}{)}_2{\mathrm{b}}_2\right]}^{nt}\\ \text{ Total structural isomers }=0\\ \text{ Total geometrical isomers }=2\\ \text{ Total optical isomers }=3\\ \text{ Total stereoisomers }=3\end{array}$
$\left(\mathrm{b}\right) {\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_5\mathrm{Cl}\right]}^{2+}\approx {\left[{\mathrm{Ma}}_5\mathrm{b}\right]}^{\pm \mathrm{n}}$
Total stmctural isomers = 0
Total geometrical isomers = 0
Total optical isomers = 0
Total stereoisomers = 0
$\text{ (c) }{\left[\mathrm{Ir}\left({\mathrm{PR}}_3\right)\mathrm{H}\left(\mathrm{CO}\right)\right]}^{2+}\approx {\left[{\mathrm{Ma}}_2\mathrm{bc}\right]}^{\mathrm{n}\pm }\text{ (Square planer) }$
Total structural isomers = 0
Total geometrical isomers = 2
Total oPtical isomers =0
Total stereoisomers = 2
$\text{ (d) }{\left[\mathrm{Ru}{\left({\mathrm{NH}}_3\right)}_4{\mathrm{Cl}}_2\right]}^{+}\approx {\left[{\mathrm{Ma}}_4{\mathrm{b}}_2\right]}^{\pm \mathrm{n}}$
Total strucnrral isomers = 0
Total geometrical isomers =2
Total optical isomers = 0
Total stereoisomers = 2