Law of multiple proportions is not applicable for the oxide(s) of:

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 - $C{O}_2,CO$
Oxides of iron - $F{e}_2{O}_3,FeO$
Oxides of nitrogen - $NO,N{O}_2,N_2{O}_5,N_2{O}_4,N_2{O}_3$
Oxides of aluminium - $A{l}_2{O}_3$ - it has only one oxide.