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

With decrease in size from Al to S the basic nature of oxide decrease and acidic nature increases.Al2O3<  SiO2< P2O3  <  SO2Al2O3 is amphoteric, SiO2 is slightly acidic whereas P2O3  and  SiO2 are the anhydrides of acids H3PO3  and  H2SO3.