Hey there!:
H is always +1 so the H's have a +3 charge.
O is always -2 so the O's have a -8 charge .
Now, suppose oxidation state for P = X , then :
+3 + X + (-8) = 0 (because of neutral molecule)
x = 8 - 3
x = + 5
So, X = +5 oxidation state.
Answer C
Hope that helps!
