The oxidation state of phosphorus is +3 in
a) orthophosphoric acid b) phosphorus acid
c) metaphosphoric acid d) phyrophosphoric acid

b) Phosphorus acid

Explanation:

To distinguish the type of acid of phosphorus with the oxidation state of +3, we need to be familiar with the chemical formula of each of the compounds:

    Orthophosphoric acid             H₃PO₄

    Phosphorus acid                       H₃PO₃

    Metaphosphoric acid               HPO₃

    Phyrophosphoric acid               H₄P₂O₇

Now that we know the formula of the given compounds, the algebraic sum of all the oxidation numbers of all atoms in a neutral compound is zero:

Only phosphorus acid yielded an oxidation state of +3 for phosphorus in the compound.

  H₃PO₃:

   we know the oxidation state of H = +1

                                                          O = -2

         The oxidation state of P is unknown. We can express this as an equation:

                3(+1) + P + 3(-2) = 0

                    3 + P -6 = 0

                          P-3 = 0

                          P = +3

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