The correct option is A Aluminium
The law of multiple proportions states that:
When two elements chemically combine to form two or more than two compounds, then the masses of one of the elements which combine with a fixed mass of the other are in a simple whole-number ratio.
Oxides of carbon - CO2,CO
Oxides of iron - Fe2O3,FeO
Oxides of nitrogen - NO,NO2,N2O5,N2O4,N2O3
Oxides of aluminium - Al2O3 - it has only one oxide.