${\mathrm{H}}_3{\mathrm{PO}}_2\stackrel{{140}^{\circ }\mathrm{C}}{⟶}\mathrm{A}\stackrel{{220}^{\circ }\mathrm{C}}{⟶}\mathrm{B}\stackrel{{320}^{\circ }\mathrm{C}}{⟶}\mathrm{C}$ Compound (C) is :

 

The reaction is,

${\mathrm{H}}_3{\mathrm{PO}}_2\stackrel{{140}^{\circ }\mathrm{C}}{⟶}\mathrm{A}\stackrel{{220}^{\circ }\mathrm{C}}{⟶}\mathrm{B}\stackrel{{320}^{\circ }\mathrm{C}}{⟶}{\left(\mathrm{HPO}3\right)}_{\mathrm{n}}$

It is an inorganic monovalent oxoanion of phosphorus. It is a base of a phosphonic acid conjugate. It is an acid conjugate of phosphonate (2-).