Water is H2O, meaning that it is bonded together which means it is not a mixture. A mixture is two substances mixed together but not bonded together. Methane is CH4, so again it is bonded together so again it is not a mixture. Magnesium is just an element so there is nothing mixed in it since it is a pure substance. So the answer is Air since it is a mixture of gases.
Pure air is a mixture of several gases that are invisible and odorless. Consists of 78% nitrogen, 21% oxygen, and less than 1% of argon, carbon dioxide.
