Concept:A solution is a homogeneous mixture of two or more substances.
Explanation:•
Alloy: It is a homogeneous mixture of two or more metals, often with non-metals. Example: steel, brass. So it is a solid solution.
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Air: It is a homogeneous mixture of gases like nitrogen, oxygen, argon, etc. This is a gaseous solution.
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Milk: It is a colloidal mixture of water, fat, proteins, and lactose. It is not a true solution because the particles are larger and do not dissolve uniformly; it shows the Tyndall effect.
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Sugar: It is a pure chemical compound (sucrose) made of carbon, hydrogen, and oxygen atoms bonded together. It is not a mixture, so it cannot be a solution.
Thus, among the given options, sugar is the only one that is a pure substance and not a solution.
Answer:Sugar