Amongst the following,the most stable complex is:
$(i)$ $[Fe(H_2O)_6]^{3+}$
$(ii)$ $[Fe(NH_3)_6]^{3+}$
$(iii)$ $[Fe(C_2O_4)_3]^{3-}$
$(iv)$ $[FeCl_6]^{3-}$

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.

The aqueous solution of which of the following complexes has the least conductivity under identical conditions?

Identify the complex with the maximum conductivity.

Assign $A, B, C, D$ from the given type of reaction:
$3KCN + Fe(CN)_3 \downarrow \longrightarrow K_3[Fe(CN)_6]$

How does ammonia react with a solution of $Cu^{2+}$?