Which of the following hydrides is the least stable?

The statement that is $NOT$ correct is:

Nitrogen dioxide is dissolved in water to produce

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:

What are the acid anhydrides of $H_3PO_4$ and $HPO_3$ respectively?

The number of $H$ atoms directly attached to the $P$ atom in hypophosphorous acid is: