Among Al₂O₃, SiO₂, P₂O₃ and SO₂ the correct order of acid strength is

As electronegativity difference between element and oxygen decreases the acidic character of oxides increases. The electronegativity also increases with increasing oxidiation state. In general, as non-metallic character increases across the period, the acidic character of their oxides increases. Again the oxides of metalloid are amphoteric and oxides of metal are basic.
Al₂O₃ ​ is amphoteric, SiO₂ ​ is slightly acidic whereas P₂O₃ ​ and SO₂ ​ are anhydrides of acid H₃PO₃ ​ and H₂SO₃.

So the correct order of acidic strength oxides of 3rd period elements is

Al₂O₃ < SiO₂ < P₂O₃ < SO₂