Taking stability as the factor,which one of the following represents the correct relationship?

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

The most stable hydride is

Why is $BiH_{3}$ the strongest reducing agent amongst all the hydrides of Group $15$ elements?

$A$ $p$-block element $(E)$ and hydrogen form a binary cation $(EH_{x}^{+})$, while $EH_{3}$ on treatment with $K_{2}HgI_{4}$ in alkaline medium gives a precipitate of basic mercury$(II)$amido-iodine. Given below are first ionisation enthalpy values $(kJ \ mol^{-1})$ for the first element of each group $13, 14, 15$, and $16$. Identify the correct first ionisation enthalpy value for element $E$.

Assertion : Molecular nitrogen is less reactive than molecular oxygen.
Reason : The bond length of $N_2$ is shorter than that of oxygen.