Correct Answer - C
Phosphonic acid `(H_(3)PO_(3))`, oxoacid of
Phosphorus shows a tendency of disproportionation. On heating `H_(3)PO_(3)`
disproportionates to give orthophosphoric acid and phoosphene gas.
`4H_(3)PO_(3)overset(Delta)to3H_(3)PO_(4)+PH_(3)`