Which of the following compounds does not exhibit optical isomerism?

Select the complex in which the metal has primary valency $= 2$,secondary valency $= 4$ and shows stereo-isomerism:

$A$ complex of the type $[M(AA)_{2}X_{2}]^{n+}$ is known to be optically active. What does this indicate about the structure of the complex? Give one example of such a complex.

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

Which of the following statements is incorrect?

Which of the following complexes will exhibit optical isomerism?