Which of the following is a tetrabasic acid?

The strongest acid among the given options is:

Why does $PCl_3$ fume in moisture?

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?

The decreasing values of bond angles from $NH_3$ $(106^o)$ to $SbH_3$ $(101^o)$ down group-$15$ of the periodic table is due to

The most stable hydride is