30 g of Magnesium and 30 g of Oxygen are reacted,
then the residual mixture contains

2Mg + O2 -> 2MgO

Explanation:

According to the above reaction ,

28.6g(2mole) Mg combine with 32g (1mole) of O2 to give 80.6g (2 mole) of MgO.

thus,

30g Mg will combine with 20g of O2 to produce 50g MgO and 10gO2 will remain unreactive.

I hope it's helpful!

i'm assuming that it's c.) red bank..