Which of the following is not optically active?

Consider the following statements and select the correct option using the codes given.
$(i)$ $[Cr(NH_3)_6][Cr(CN)_6]$ and $[Cr(NH_3)_4(CN)_2][Cr(NH_3)_2(CN)_4]$ are coordination isomers.
$(ii)$ $[Cr(py)_2(H_2O)_2Cl_2]Cl$ and $[Cr(py)_2(H_2O)Cl_3]H_2O$ are linkage isomers.
$(iii)$ $[Pt(NH_3)_4Br_2]Cl_2$ and $[Pt(NH_3)_4Cl_2]Br_2$ are linkage isomers.
$(iv)$ $[NiCl_2(PPh_3)_2]$ (tetrahedral) exhibits geometrical isomerism.

In which of the following complex species,$\Delta_0 > P$ (pairing energy) with magnetic moment $\sqrt{3} \ B.M.$?

Identify a $CORRECT$ formula for spin only magnetic moment.

The number of paramagnetic species from the following is $............$.
$[Ni(CN)_4]^{2-}, [Ni(CO)_4], [NiCl_4]^{2-}$
$[Fe(CN)_6]^{4-}, [Cu(NH_3)_4]^{2+}$
$[Fe(CN)_6]^{3-}$ and $[Fe(H_2O)_6]^{2+}$

Which of the following complexes will be optically inactive?