Which of the following best describes a compound?

A compound is defined as a substance formed when two or more different elements are chemically bonded together. This definition highlights that compounds consist of distinct elements joined through chemical bonds, resulting in a new substance with properties different from those of the individual elements.

For example, when hydrogen and oxygen chemically bond, they form water (H2O), which is a compound that has unique physical and chemical properties, such as boiling point and reactivity, which differ significantly from the properties of hydrogen and oxygen alone. This encompassing definition clarifies why option A correctly describes a compound.

The other options do not accurately represent the definition of a compound. A compound cannot be separated by physical means, unlike mixtures, which can typically be separated through physical processes such as filtration or distillation. Additionally, the idea of a compound being the same as a mixture is incorrect, as mixtures involve a physical blend of substances without chemical bonding. Lastly, a single element in its natural state does not constitute a compound; it refers to an individual element, which lacks the combination of different elements required to form a compound.