Which of the following reagents gives the carbylamine test when heated with $KOH$ and a primary amine?

Benzylamine $(C_6H_5CH_2NH_2)$ and aniline $(C_6H_5NH_2)$ can be distinguished by:

Nessler's reagent is

An aromatic compound "$A$" on treatment with $Zn / NH_4Cl$ and subsequent heating with ammoniacal silver nitrate solution gives a black precipitate. Which functional group does "$A$" contain?

Assertion: Benzene diazonium chloride does not give a test for nitrogen.
Reason: Loss of $N_2$ gas takes place during heating.

Compound $P$ is neutral. $Q$ gives effervescence with $NaHCO_3$ while $R$ reacts with Hinsberg's reagent to give a solid soluble in $NaOH$. Compound $P$ is: