Among ${\mathrm{Al}}_2{\mathrm{O}}_3,{\mathrm{SiO}}_2,{\mathrm{P}}_2{\mathrm{O}}_3$ and ${\mathrm{SO}}_3$ the correct order of acidic strength is :

The acidic nature of oxides grows as the electronegativity difference between the element and oxygen decreases. As the oxidation state increases, so does the electronegativity. In general, the acidic character of their oxides increases as non-metallic character grows over time.
Thus, the proper listing of acidic strengths is as follows:

${\mathrm{Al}}_2{\mathrm{O}}_3<{\mathrm{SiO}}_2<{\mathrm{P}}_2{\mathrm{O}}_3<{\mathrm{SO}}_2$.