Which one of the following is an inner orbital complex as well as diamagnetic in behaviour? (Atomic number: $Zn = 30$,$Cr = 24$,$Co = 27$,$Ni = 28$)

Hybridizations of the central atom in $[Fe(H_2O)_6]^{3+}$ and $[Co(NH_3)_6]^{3+}$,respectively,are

Assertion $:$ Complex ion $[Co(NH_3)_6]^{2+}$ is readily oxidized to $[Co(NH_3)_6]^{3+}$.
Reason $:$ Unpaired electron in complex ion $[Co(NH_3)_6]^{2+}$ is present in $4p$ orbital.

The hybridisation of $Ni$,shape,and number of unpaired electrons present in $[NiCl_4]^{2-}$ are respectively:

Which of the following statements are true about $[NiCl_4]^{2-}$?
$(a)$ The complex has tetrahedral geometry
$(b)$ Coordination number of $Ni$ is $2$ and oxidation state is $+4$
$(c)$ The complex is $sp^3$ hybridised
$(d)$ It is a high spin complex
$(e)$ The complex is paramagnetic

Discuss the complex species $[Co(NH_3)_6]^{3+}$ based on Valence Bond Theory.