Which of the following describes a compound?

A compound is formed when two or more elements chemically combine in fixed ratios, resulting in the creation of a substance with unique properties distinct from those of its individual components. This formation process occurs during a chemical change, where bonds between atoms are formed or broken, leading to the creation of a new substance. Unlike mixtures, which can consist of varying amounts of different substances and retain the individual properties of their components, compounds exhibit properties that are not similar to those of the elements they are made from.

In essence, the definition of a compound emphasizes the importance of chemical reactions in its formation, highlighting that it cannot merely be separated into its components through physical means, nor does it consist of only one type of particle or possess characteristics identical to its parts. Thus, the choice that states compounds are formed during a chemical change accurately captures the fundamental nature of what a compound is.