Assign $A, B, C, D$ from the given type of reaction: $Cd(CN)_2 \downarrow + 2KCN \longrightarrow K_2[Cd(CN)_4]$

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?

When $CuSO_4$ solution in water is treated with concentrated $HCl$,it turns:

$CuSO_4$ solution reacts with excess of $KCN$ solution to form:

The $\pi-$acid ligand which uses its $d-$orbital during synergic bonding in its complex compound is:

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.