Which among the following cations forms a least stable complex if the ligand is same?

$A$ solution contains $2.675 \ g$ of $CoCl_3 \cdot 6NH_3$ $(M = 267.5 \ g/mol)$. When passed through a cation exchanger,the chloride ions liberated are treated with excess $AgNO_3$ to yield $4.78 \ g$ of $AgCl$ $(M = 143.5 \ g/mol)$. What is the formula of the coordination compound?

Copper sulphate solution reacts with $KCN$ to give

Which one of the following metal complexes is most stable?

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 active substance in Nessler's reagent used for the detection of ammonia is ..........