With which of the following will ammonia $NOT$ form a complex compound?

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.

In an ammoniacal solution,copper ions form the complex ion $[Cu(NH_3)_4]^{2+}$,but this does not occur in an acidic solution. The reason for this is:

Solubility of iodine in water is greatly increased by the addition of iodide ions because of the formation of ......

Which of the following complexes is the most stable?

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