Phosphorus acid (H₃PO₃) is a diprotic acid. This means it has two acidic hydrogens that can be donated.

Here's why:

* Structure: The structure of phosphorus acid has two hydrogen atoms directly attached to the phosphorus atom, and one hydrogen atom attached to an oxygen atom.

* Acidic Hydrogens: The two hydrogen atoms attached to the phosphorus atom are the acidic ones. They are readily released as protons (H⁺) in solution.

* Non-acidic Hydrogen: The hydrogen atom attached to the oxygen atom is not acidic.

Therefore, the basicity of phosphorus acid is 2.