The correct order of acidic strength is
SO2>Cl2O7>P4O10
Cl2O7>SO2>P4O10
P4O10>SO2>Cl2O7
N2O5>P4O10>SO2
Oxidation state of central atom in oxide is directly proportional to acidity of oxides. Hence the correct order of acidic strength is Cl2O7>SO2>P4O10