The correct option is C Nitric Acid
The phosphorus present in an organic compound is oxidized to phosphoric acid by heating with fuming nitric acid. The phosphoric acid so obtained is precipitated as MgNH4PO4MgNH4PO4, which on ignition is converted into Mg2P2O7Mg2P2O7. From the mass of Mg2P2O7Mg2P2O7, the mass of phosphorus present in the sample of the organic compound can be calculated.