Which among the following group $15$ elements does not exhibit allotropy?

Amongst the following compounds:
$I.$ $H_5P_3O_{10}$
$II.$ $H_6P_4O_{13}$
$III.$ $H_5P_5O_{15}$
$IV.$ $H_7P_5O_{16}$
Which of these are non-cyclic phosphates?

Which of the phosphorus oxoacids can create a silver mirror from $AgNO_3$ solution?

$PCl_5$ exists but $NCl_5$ does not because:

Ionic radius (in $\mathring{A}$) of $As^{3+}$,$Sb^{3+}$ and $Bi^{3+}$ follow the order:

$A$ nitrogen oxide that forms "in situ" when dilute $FeSO_4$ is treated with an aqueous solution of nitrate ion and then carefully adding conc. $H_2SO_4$ along the sides of the test tube, is