Which one of the following complexes is violet in colour?

Nitrate is detected by the ring test. The brown color of the ring is due to which of the following?

The stepwise formation of $[Cu(NH_{3})_{4}]^{2+}$ is given below:
$Cu^{2+} + NH_{3} \rightleftharpoons [Cu(NH_{3})]^{2+} \quad K_{1}$
$[Cu(NH_{3})]^{2+} + NH_{3} \rightleftharpoons [Cu(NH_{3})_{2}]^{2+} \quad K_{2}$
$[Cu(NH_{3})_{2}]^{2+} + NH_{3} \rightleftharpoons [Cu(NH_{3})_{3}]^{2+} \quad K_{3}$
$[Cu(NH_{3})_{3}]^{2+} + NH_{3} \rightleftharpoons [Cu(NH_{3})_{4}]^{2+} \quad K_{4}$
The values of stability constants $K_{1}, K_{2}, K_{3}$ and $K_{4}$ are $10^{4}, 1.58 \times 10^{3}, 5 \times 10^{2}$ and $10^{2}$ respectively. The overall equilibrium constant for dissociation of $[Cu(NH_{3})_{4}]^{2+}$ is $x \times 10^{-12}$. The value of $x$ is ...............
(Rounded off to the nearest integer)

The complex formed during the brown ring test of nitrate $(NO_3^-)$ ion is:

Assertion : $NF_3$ is a weaker ligand than $N(CH_3)_3$.
Reason : $NF_3$ ionizes to give $F^{-}$ ions in aqueous solution.

Urea reacts with water to form $A$ which will decompose to form $B$. $B$ when passed through $Cu^{2+}_{(aq)}$,a deep blue coloured solution $C$ is formed. What is the formula of $C$ from the following?