What is the active species in Nessler's reagent used for the identification of ammonia?

Which of the following oxides is the least acidic?

In which oxidation state,group $15$ elements act as Lewis base?

What is the correct order of acidic character for the given oxides?

The gas produced by treating an aqueous solution of ammonium chloride with sodium nitrite is .... .

Find the correct statements related to group $15$ hydrides.
$A$. Reducing nature increases from $NH_3$ to $BiH_3$
$B$. Tendency to donate lone pair of electrons decreases from $NH_3$ to $BiH_3$
$C$. The stability of hydrides decreases from $NH_3$ to $BiH_3$
$D$. $HEH$ bond angle decreases from $NH_3$ to $SbH_3$ ($E$ = Elements of group $15$)
Choose the correct answer from the options given below: