which of the following yields oxide with the formula MO by rxn w/ oxygen?

a. sodium
b. potassium
c. aluminum
d. magnesium
e. lithium

I know it 's c or d for sure. But doesn't Al have +2 oxidation number.

Which lead me to another q. How do you know which oxidization number to use to form a compound if the metal has more than one oxidation number (i.e. Copper)


thanks!