A compound is a pure substance made up of two or more element in fixed amount.

Compound is a pure substance made up of two or more elements. When multiple elements chemically combine, they form a compound with distinct properties different from the original elements. The correct answer to the question is option 'D' - a compound is a pure substance made up of two or more elements.

Explanation:
- Definition of a Compound: A compound is a substance that is made up of two or more different elements that are chemically bonded together. The elements in a compound are present in a fixed ratio and cannot be separated by physical means.
- Pure Substance: A pure substance is a form of matter that has a definite composition and distinct properties. Compounds are examples of pure substances because they have a specific composition and their properties are different from the elements that make them up.
- Elements: Elements are substances that cannot be broken down into simpler substances by chemical means. They consist of only one type of atom. Each element is represented by a unique chemical symbol.
- Chemical Bonding: When two or more elements chemically combine, they form a compound through a process called chemical bonding. During chemical bonding, the atoms of the elements rearrange their outermost electrons to achieve a stable electron configuration. This results in the formation of new chemical bonds between the atoms.
- Distinct Properties: Compounds have properties that are different from the properties of the individual elements that make them up. For example, table salt (sodium chloride) is a compound made up of the elements sodium and chlorine. Sodium is a highly reactive metal, while chlorine is a toxic gas. However, when they combine to form sodium chloride, a compound, the resulting substance is a white, crystalline solid that is safe to consume.
- Examples: Some common examples of compounds include water (H2O), carbon dioxide (CO2), ammonia (NH3), and glucose (C6H12O6). In each of these compounds, two or more elements are chemically bonded together to form a new substance with unique properties.

In conclusion, a compound is a pure substance made up of two or more elements that are chemically bonded together. Compounds have distinct properties different from the elements that make them up.