Which of the following complexes shows the maximum molar conductivity in aqueous solution?

Given below are two statements: one is labelled as "Assertion $A$" and the other is labelled as "Reason $R$".
Assertion $A$: In the complexes $Ni(CO)_4$ and $Fe(CO)_5$,the metals have zero oxidation state.
Reason $R$: Low oxidation states are found when a complex has ligands capable of $\pi$-donor character in addition to the $\sigma$-bonding.
In the light of the above statements,choose the most appropriate answer from the options given below.

$CuSO_4$ solution reacts with excess $KCN$ to give

$A$ solution containing $2.675 \ g$ of $CoCl_3 \cdot 6 NH_3$ (molar mass $= 267.5 \ g \ mol^{-1}$) is passed through a cation exchanger. The chloride ions obtained in solution were treated with excess of $AgNO_3$ to give $4.78 \ g$ of $AgCl$ (molar mass $= 143.5 \ g \ mol^{-1}$). The formula of the complex is
(At. mass of $Ag = 108 \ u$)

Assign $A$,$B$,$C$,$D$ from the given type of reaction.
$HgI_2 \downarrow + 2KI \longrightarrow K_2[HgI_4]$

The correct increasing order of stability of the complexes based on $\Delta_o$ value is $:$
$I. \ [Mn(CN)_6]^{3-}$
$II. \ [Co(CN)_6]^{4-}$
$III. \ [Fe(CN)_6]^{4-}$
$IV. \ [Fe(CN)_6]^{3-}$