The correct option is D Orthophosphorous acid
Oxidation state of hydrogen = +1
Oxidation state of oxygen = −2
In Orthophosphoric acid (H3PO4), phosphorus has the oxidation state of +5.
In Orthophosphorus acid (H3PO3), phosphorus has the oxidation state of +3.
In metaphosphoric acid (HPO3), phosphorus has the oxidation state of +5.
In pyrophosphoric acid (H4P2O7), phosphorus has the oxidation state of +5.
Hence, option (c) is correct.