( a ) H₃PO₃
( b ) PCl₃
( c ) Ca₃P₂
( d ) Na₃PO₄
( e ) POF₃?

Let the oxidation state of phosphorous in each case be y.
(a) H₃PO₃
3 + y + 3( -2) = 0
y -3 = 0
y =3

(b) PCl₃
y + 3( -1) = 0
y = 3

(c) Ca₃P₂
3( 2 ) + 2 (y) = 0
2y = -6
y = -3

(d) Na₃PO₄
3( 1 ) + y + 4( -2 ) = 0
y -5 =0
y =5

(e)POF₃
y + ( -2 ) + 3( -1) = 0
y -5 = 0
y = 5