Which of the following is the most stable complex ion?

$X$ is the number of electrons in $t_{2g}$ orbitals of the most stable complex ion among $[Fe(NH_3)_6]^{3+}$,$[Fe(Cl)_6]^{3-}$,$[Fe(C_2O_4)_3]^{3-}$ and $[Fe(H_2O)_6]^{3+}$. The nature of the oxide of vanadium of the type $V_2O_X$ is:

Assign $A, B, C, D$ from the given type of reaction.
$CuSO_4 + NH_3$ (Excess) $\longrightarrow [Cu(NH_3)_4]^{2+}$

If $1/3$ of the total chlorine in the compound is precipitated on adding $AgNO_3$ to its aqueous solution,which of the following is the correct structure of $CrCl_3 \cdot 6H_2O$?

Which among the following is a correct decreasing order of thermodynamic stability of the complexes?

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 $\left(N_A = 6 \times 10^{23} \ mol^{-1}\right)$