In which substance does phosphorus have a +3 oxidation state

Options are as follow,

A. P₄O₁₀
B. PCl₅
C. Ca₃(PO₄)₂
D. KH₂PO₃

Answer;
            The correct option is Option-D (KH₂PO₃).

Explanation:
                   Oxidation number of Phosphorous is calculated as,

As the normal oxidation number of K is +1, H is +1 and O is -2 so,

                                       KH₂PO₃  =  0

0 shows that the molecule is neutral. Now puttion values of all elemnts except P,
 
                                  (+1) + (+1)₂ + P + (-2)₃  =  0

                                  +1 + 2 + P - 6  =  0

                                  +3 + P - 6  =  0

                                  +3  + P  =  +6

                                  P  =  +6 - 3

                                  P  =  +3