Which element of group $15$ forms the most basic compound with hydrogen?

Which of the following compounds is a tribasic acid?

Which of the following oxides is the anhydride of orthophosphoric acid?

Phosphine is prepared by the reaction of $P_4$ with which of the following?

Which of the following oxides of nitrogen is a coloured gas?

$A$ obtained by Ostwald's method involving air oxidation of $NH_3$,upon further air oxidation produces $B$. $B$ on hydration forms an oxoacid of Nitrogen along with evolution of $A$. The oxoacid also produces $A$ and gives positive brown ring test.