Final answer:
Sugar is a compound composed of carbon, hydrogen, and oxygen, whereas air is a mixture, steel is an alloy, and nitrogen is an element. So the correct answer is D.
Step-by-step explanation:
A compound is a substance composed of two or more elements that are chemically combined in fixed ratios. Let's analyze the given options:
Air: Air is a mixture of various gases, primarily nitrogen, oxygen, carbon dioxide, and others. It is not a compound because its components are not chemically bonded in fixed ratios.
Steel: Steel is an alloy composed mainly of iron and carbon, along with other elements like manganese and chromium. While it is a combination of elements, it is not a compound because the elements are not bonded together in a specific chemical ratio.
Nitrogen: Nitrogen is a diatomic molecule (N2), consisting of two nitrogen atoms chemically bonded together. It is a compound because it meets the definition of a chemical compound with a specific composition.
Sugar: Sugar refers to a group of compounds, including glucose and sucrose. These compounds are made up of carbon, hydrogen, and oxygen atoms bonded together in specific ratios, making sugar a compound.
Therefore, the correct answer is D. sugar. It is a compound due to its specific chemical composition, while the other options are either mixtures or alloys.