Answer:
Atmospheric air
Explanation:
Atmospheric air is a mixture of nitrogen, oxygen, argon, and small amounts of other gases.
A is wrong. A flask of oxygen atoms has nothing else in it. The O atoms are a pure substance.
B is wrong. A flask of distilled water contains only water. The water is a pure substance.
D is wrong. A flask of ozone contains only ozone. Ozone is a pure substance,
