Answer is: C. H3PO4, phosphoric acid.
A) HClO₂ is chlorous acid.
Chlorous acid is weak inorganic acid. In this acid, chlorine has oxidation number +3.
B) HNO₂ is nitrous acid.
Nitrous acid is a weak monobasic acid. In this acid, nitrogen has oxidation number +3.
D) HI is hydroiodic acid.
Hydroiodic acid is a highly acidic aqueous solution of hydrogen iodide. In this acid, iodine has oxidation number -1.