Complexes of $CN^-$ ligand are generally...

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.

Which of the following has the highest molar conductivity in solution?

Assign $A, B, C, D$ from the given type of reaction: $AgI \downarrow + 2Na_2S_2O_3 \longrightarrow Na_3[Ag(S_2O_3)_2] + NaI$

When $0.1 \ mole$ of $CoCl_3(NH_3)_5$ is treated with excess of $AgNO_3$,$0.2 \ mole$ of $AgCl$ are obtained. The conductivity of the solution will correspond to:

Which of the following is the most stable complex?