Which of the following statements illustrates the law of multiple proportions?

In XO, 50 g of element combines with 50 g of oxygen.
$\therefore$1 g of element combines with 1 g of oxygen.
In XO₂, 40 g of element combines with 60 g of oxygen.
$\therefore$1 g of element combines with 1.5 g of oxygen.
Thus, ratio of masses of oxygen which combines with 1 g of element is 1 : 1.5 or 2 : 3. This is in accordance with the law of multiple proportions. 

In second option, Gay Lussac's law of gaseous volume is followed. 

In third option, law of conservation of mass is followed while in fourth option, Avogadro's law is followed.