Which of the following complex ions does not absorb visible light?

The species that can have a $trans$-isomer is: ($en = \text{ethane-}1,2\text{-diamine}$,$ox = \text{oxalate}$)

Which of the following represents a pair of enantiomers?

The total number of geometrical isomers possible for an octahedral complex of the type $[MA_{2}B_{2}C_{2}]$ is ($M=$ transition metal; $A, B$ and $C$ are monodentate ligands).

Amongst $[Ni(H_{2}O)_{6}]^{2+}$,$[Ni(PPh_{3})_{2}Cl_{2}]$,$[Ni(CO)_{4}]$ and $[Ni(CN)_{4}]^{2-}$,the paramagnetic species are

Which of the following is a chiral complex? Here $en = \text{ethylenediamine}$.