Nitrogen combines with metals to form

Which of the following tendencies remains unchanged on going down in the nitrogen family (Group-$15$)?

$P_4 + 3NaOH + 3H_2O \to A + 3NaH_2PO_2$. Here,$A$ is:

The following are some statements related to $VA$ group hydrides:
$I$. Reducing property increases from $NH_3$ to $BiH_3$.
$II$. Tendency to donate lone pair decreases from $NH_3$ to $BiH_3$.
$III$. Thermal stability of hydrides decreases from $NH_3$ to $BiH_3$.
$IV$. Bond angle of hydrides decreases from $NH_3$ to $BiH_3$.
The correct statements are:

$NH_3 + O_2 \xrightarrow{\text{Catalyst}} NO + H_2O$. This step is used in which process?

The strongest acid among the given options is: