Which of the following is the correct order of stability of 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 $E$.

The total number of $\sigma$ and $\pi$-bonds in pyrophosphoric acid are respectively

Which of the following substances is the most acidic?

$A$ soft heavy metal melts at $30\ ^oC$ and is used in making heat-sensitive thermometers. The metal is:

Reaction of thionyl chloride with white phosphorus forms a compound $[A]$,which on hydrolysis gives $[B]$,a dibasic acid. $[A]$ and $[B]$ are respectively