Oxidation State Order of Phosphorus Oxyacids

To determine the correct decreasing order of the oxidation state of phosphorus ($\text{P}$) in the given oxyacids, we calculate the oxidation number of $\text{P}$ in each molecule using standard oxidation state rules (Hydrogen = $+1$, Oxygen = $-2$):

1. Orthophosphoric acid ($\text{H}_3\text{PO}_4$): $3(+1) + x + 4(-2) = 0$ $3 + x - 8 = 0 \implies x = +5$

2. Hypophosphoric acid ($\text{H}_4\text{P}_2\text{O}_6$): $4(+1) + 2x + 6(-2) = 0$ $4 + 2x - 12 = 0 \implies 2x = 8 \implies x = +4$

3. Orthophosphorous acid ($\text{H}_3\text{PO}_3$): $3(+1) + x + 3(-2) = 0$ $3 + x - 6 = 0 \implies x = +3$

4. Hypophosphorous acid ($\text{H}_3\text{PO}_2$): $3(+1) + x + 2(-2) = 0$ $3 + x - 4 = 0 \implies x = +1$

Comparing the calculated oxidation states of phosphorus: $\text{H}_3\text{PO}_4 (+5) > \text{H}_4\text{P}_2\text{O}_6 (+4) > \text{H}_3\text{PO}_3 (+3) > \text{H}_3\text{PO}_2 (+1)$

Thus, the correct decreasing sequence of the oxidation state of phosphorus is: $\text{H}_3\text{PO}_4 > \text{H}_4\text{P}_2\text{O}_6 > \text{H}_3\text{PO}_3 > \text{H}_3\text{PO}_2$

This matches Option C.