The correct option is C P4O10<SO3<Cl2O7
The oxides of metals are basic in nature whereas, the oxides of non-metals are acidic in nature. Also, the oxides of metalloids are amphoteric in nature.
Non-metallic character increases as we move from left to right across the period. So, the acidic strength of their oxides will also increase. Therefore, the correct order of acidic strengths is: P4O10<SO3<Cl2O7