Which of the following exhibits coordination isomerism?

The calculated spin-only magnetic moments of $[Cr(NH_3)_6]^{3+}$ and $[CuF_6]^{3-}$ in $BM$,respectively,are (Atomic numbers of $Cr$ and $Cu$ are $24$ and $29$,respectively).

The observed magnetic moment of the complex $[Mn(NCS)_6]^{x-}$ is $6.06 \ BM$. The numerical value of $x$ is $.......$.

Among $[Ni(CO)_4]$,$[NiCl_4]^{2-}$,$[Co(NH_3)_4Cl_2]Cl$,$Na_3[CoF_6]$,$Na_2O_2$ and $CsO_2$,the total number of paramagnetic compounds is

The pair in which both the species have the same magnetic moment (spin only) is

Which of the following complexes exhibits optical isomerism?