The incorrect statement about $Ni(CO)_4$ is

The geometry of $[Ni(CO)_4]$ and $[NiCl_2(PPh_3)_2]$ are

For which of the following $d^n$ configurations of octahedral complexes,can they $NOT$ exist in both high spin and low spin forms?
$(I)$ $d^3$,$(II)$ $d^5$,$(III)$ $d^6$,$(IV)$ $d^8$

The pair$(s)$ of complexes wherein both exhibit tetrahedral geometry is(are):
Note: $py = \text{pyridine}$
Given: Atomic numbers of $Fe, Co, Ni$ and $Cu$ are $26, 27, 28$ and $29$,respectively.
$(A)$ $[FeCl_4]^-$ and $[Fe(CO)_4]^{2-}$
$(B)$ $[Co(CO)_4]^-$ and $[CoCl_4]^{2-}$
$(C)$ $[Ni(CO)_4]$ and $[Ni(CN)_4]^{2-}$
$(D)$ $[Cu(py)_4]^+$ and $[Cu(CN)_4]^{3-}$

The magnetic moment for two complexes of empirical formula $Ni(NH_3)_4(NO_3)_2 \cdot 2H_2O$ is $0$ and $2.84 \ BM$ respectively. The second complex is not a neutral complex. The correct formula and geometry of the first complex is:

Which of the following complexes has $dsp^2$ hybridisation?