Ordinary light is converted into plane-polarized light with the help of which of the following?

Which of the following coordination compounds would exhibit optical isomerism?

$A$ complex $[CoL_6]^{n+}$ where $L$ is a neutral ligand has a magnetic moment $\mu = 4.5 \ BM$. Hence,

The 'spin only' magnetic moment value of $MO_4^{2-}$ is . . . . . $BM$. (Where $M$ is a metal having least metallic radii among $Sc, Ti, V, Cr, Mn$ and $Zn$).
(Given atomic number: $Sc=21, Ti=22, V=23, Cr=24, Mn=25$ and $Zn=30$)

The spin-only magnetic moment value for the complex $[Co(CN)_6]^{4-}$ is $...... \ BM$ [At. no. of $Co=27$]

Which of the following compounds is not coloured?