The oxidation number of Phosphorus (P) is the unknown here. Let us consider it as x.
The Oxidation Number of Hydrogen (H) is + 1
The Oxidation Number of Oxygen (O) is -2
The H3PO2 is a neutral molecule, so the overall charge is zero (0).
Using the values in the formula,
H3PO2 = 3(1) + x + 2(-2) = 0
H3PO2 = 3 + x -4 = 0
x -1 = 0
x = 1
The Oxidation Number of P in H3PO2 is + 1.
To confirm, substitute the value obtained in the formula, the answer should be zero as the charge on the molecule is zero.
H3PO2 = 3(1) + 1 + 2(-2) = 4-4 = 0