For which of the following configurations is the spin-only magnetic moment value $2.84 \ BM$?

The magnetic moment of a complex ion is $2.83 \ BM$. The complex ion is

Which of the following statements is $correct$?

The spin-only magnetic moment value of $M^{n+}$ ion formed among $Ni$, $Zn$, $Mn$, and $Cu$ that has the least enthalpy of atomisation is $........$ (in nearest integer). Here $n$ is equal to the number of diamagnetic complexes among $K_2[NiCl_4]$, $[Zn(H_2O)_6]Cl_2$, $K_3[Mn(CN)_6]$, and $[Cu(PPh_3)_3I]$.

The magnetic moment of the complex $[Cr(CN)_6]^{3-}$ is ..... (in $BM$)

Which of the following square planar complex molecular formulas exhibits $cis-trans$ isomerism? ($a$ and $b$ are monodentate ligands)