Which of the following oxoacids of phosphorus is the strongest reducing agent?

Solution

1. The reducing character of phosphorus oxoacids depends on the number of $\text{P-H}$ bonds present.
2. ${\text{H}}_3{\text{PO}}_2$ (hypophosphorous acid) has two $\text{P-H}$ bonds.
3. ${\text{H}}_3{\text{PO}}_3$ has one $\text{P-H}$ bond, and ${\text{H}}_3{\text{PO}}_4$ has zero $\text{P-H}$ bonds.
4. More $\text{P-H}$ bonds result in stronger reducing properties.
5. Hence the answer is (C).