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The concept of a pure substance refers to a material that has a consistent composition and distinct properties throughout. Water, often considered a quintessential pure substance, is made up entirely of H2O molecules. It does not contain any other components or mixtures, making it a clear example of a pure substance.

In contrast, saline solution is a mixture of water and salt, and air is a mixture of various gases, including oxygen, nitrogen, and carbon dioxide. Bases, while they can be pure substances (like sodium hydroxide), are generally not classified as such when referred to collectively because they can exist in mixed forms or solutions. The focus should be on the specific context in which bases are discussed, as they might not always represent a single substance with uniform properties.

Thus, water stands out as the correct classification of a pure substance, possessing consistent chemical and physical attributes that set it apart from mixtures or compounds with varying compositions.