Which of the following complex ions is the most stable?

Assign $A, B, C, D$ for the type of observation in the following reaction:
$CuSO_4 + KCN \text{ (excess)} \longrightarrow K_3[Cu(CN)_4] + \frac{1}{2}(CN)_2$

The stability constants for complexes formed by a metal ion $(M^{2+})$ with ligands $NH_3$,$CN^-$,$H_2O$,and $en$ are $10^{11}$,$10^{27}$,$10^{15}$,and $10^8$ respectively. Based on these values,which of the following is correct?

When $100 \ mL$ of $0.2 \ M$ solution of $CoCl_3 \cdot x NH_3$ is treated with excess of $AgNO_3$ solution,$3.6 \times 10^{22}$ ions are precipitated. The value of $x$ is $\left(N_A = 6 \times 10^{23} \ mol^{-1}\right)$

Which of the following has the maximum molar conductivity?

The strongest $CO$ bond is present in