How do you account for the reducing behaviour of $H_{3}PO_{2}$ on the basis of its structure?

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A)$ : $PH_3$ has lower boiling point than $NH_3$.
Reason $(R)$ : In liquid state $NH_3$ molecules are associated through van der Waals forces,but $PH_3$ molecules are associated through hydrogen bonding.
In the light of the above statements,choose the most appropriate answer from the options given below:

White phosphorus when boiled with a strong solution of caustic soda produces:

Which of the following oxides of nitrogen is the anhydride of nitrous acid?

At what temperature is white phosphorus heated to form $\beta$-black phosphorus (in $K$)?

Which of the following exhibits the highest solubility in water?