Which of the following is the structural formula of hypophosphorous acid?

What are $X$ and $Y$, respectively in the following reactions?
$Au + \text{aqua regia} \longrightarrow AuCl_4^{-} + H_2O + X$
$Pt + \text{aqua regia} \longrightarrow PtCl_6^{2-} + H_2O + Y$

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

Nitric acid is an oxidising agent and reacts with $PbO$ but it does not react with $PbO_{2}$. Explain why?

$A$ colourless,neutral,paramagnetic oxide of nitrogen '$P$' on oxidation gives reddish brown gas $Q$. $Q$ on cooling gives colourless gas $R$. $R$ on reaction with $P$ gives blue solid $S$. Identify $P, Q, R, S$ respectively.

$PH_3$ forms bubbles when passed slowly in water but $NH_3$ dissolves. Explain why?