Which of the comparison regarding $Zn,\,Cd,\,Hg$ is/are incorrect ?

$(I)$ $ZnCl_2$ is ionic whereas $CdCl_2$ and $HgCl_2$ are covalent

$(II)$ $Zn$ and $Cd$ dissolves in dilute acid $(HCl)$ liberating $H_2$ but $Hg$ can not

$(III)$ $Zn$ and $Cd$ forming with ppt. of $Zn(OH)_2$ and $Cd(OH)_2$ but $Hg$ forms coloured ppt. of

$(IV)$ all from $A_2^{2+}$ type ion

How would you account for the following:

$(i)$ of the $d^{4}$ species, $Cr ^{2+}$ is strongly reducing while manganese $(III)$ is strongly oxidising.

$(ii)$ Cobalt $(II)$ is stable in aqueous solution but in the presence of complexing reagents it is easily oxidised.

$(iii)$ The $d^{1}$ configuration is very unstable in ions.

Explain the variations in atomic radii of transition elements along the period.

In a reaction the ferrous $(F{e^{ + 2 }})$ iron is oxidised to ferric $(F{e^{ + 3 }})$ ion. The equivalent weight of the ion in the above reaction is equal to

A blue colouration is not obtained when

Given below are two statements, one is labelled as Assertion $A$ and the other is labelled as Reason $R$.

Assertion $A:$ The spin only magnetic moment value for $\left[ Fe ( CN )_6\right]^{3-}$ is $1.74 BM$, whereas for $\left[ Fe \left( H _2 O \right)_6\right]^{3+}$ is $5.92\,BM$.

Reason $R :$ In both complexes, $Fe$ is present in +3 oxidation state.

In the light of the above statements, choose the correct answer from the options given below: