Check option A: Compare $H_3PO_3$ and $H_2SO_3$.

• $H_3PO_3$ (phosphorous acid) is a dibasic acid with structure $HPO(OH)_2$.
• $H_2SO_3$ (sulfurous acid) is also a weak acid.
• In general, oxyacid strength depends on factors like electronegativity of the central atom and oxidation state.
• Sulfur in $H_2SO_3$ is in oxidation state $+4$, phosphorus in $H_3PO_3$ is in $+3$.
• Sulfur is more electronegative than phosphorus, so $H_2SO_3$ is stronger than $H_3PO_3$.

Therefore, A is false.

Check option B: Compare HF and HCl in aqueous medium.

• In water, $HCl$ is a strong acid and dissociates almost completely.
• HF is a weak acid due to the very strong $H-F$ bond.

Therefore, B is false.

Check option C: Compare $HClO_4$ and $HClO_3$.

• For oxyacids of the same halogen, acid strength increases with the number of oxygen atoms.
• Thus, $HClO_4 > HClO_3$ in acid strength.

So the statement "$HClO_4$ is weaker than $HClO_3$" is wrong.

Therefore, C is false.

Check option D: Compare $HNO_3$ and $HNO_2$.

• For oxyacids of the same central atom, more oxygen atoms increase acid strength because they stabilize the conjugate base by resonance and exert stronger $-I$ effect.
• Hence, $HNO_3 > HNO_2$ in acid strength.

Therefore, D is true.

Final conclusion:

The true statement is: $\boxed{D}$

Comparison with stored answer:

Stored correct answer = $D$.

My derived answer also = $D$, so they agree.