Element X forms five stable oxides of formula - $X_{2}O,XO,X_2{O}_3,X_2{O}_4,X_2{O}_5$. The formation of the oxides explains:

The correct option is C Law of multiple proportions
Law of multiple proportions states that if two elements form more than one compound between them, then the ratios of the masses of the second element which combines with the fixed mass of the first element will always be a ratio of small whole numbers
On fixing the mass of oxygen, X reacts with oxygen in different proportions thus explaining law of multiple proportions.