Among Al2O3, SiO2, P2O3, SO2 the correct order of acid strength is
SO2<P2O3<SiO2<Al2O3
Al2O3< SiO2<P2O3< SO2
Al2O3< SiO2< SO2<P2O3
SiO2< SO2< Al2O3< P2O3
As non-metallic character increase from Aluminium to Sulphur the acidic characters of oxides increases in the order Al,Si,P,S.