The question does not ask what happens to the percentage of molecules that contain oxygen atoms. It asks what happens to the percentage of oxygen. Although there are oxygen atoms in every water molecule, they do not, in any way, behave like oxygen. Water is a liquid. Oxygen is a gas. You cannot breathe water. You must breathe oxygen. Even fish cannot breathe water. They breathe oxygen gas which has been dissolved in water. Oxygen is required in order for something to burn. Water puts out something that is burning.

Thus, water in the air does not add to the oxygen content, because the oxygen atoms in water do not make oxygen. They are a part of the water molecules.