Which of the following oxoacids of phosphorus has the strongest reducing property?

Which of the following oxyacids has an $X-O-X$ linkage?

$A$ $p$-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 $E$.

Draw the structures of different oxoacids of phosphorus and indicate the oxidation state of phosphorus,characteristic bonds,and their numbers in each.

Reaction of ammonia with excess $Cl_2$ gives:

$(NH_4)_2Cr_2O_7$ on heating gives: