Which concept explains that a compound has a unique composition?

The correct choice is the Law of Definite Composition, which states that a chemical compound always contains the same proportion of elements by mass, regardless of its source or the amount produced. This principle highlights that a specific compound, like water (H₂O) or carbon dioxide (CO₂), will always exhibit a fixed ratio of its constituent elements.

For instance, water will always be comprised of two hydrogen atoms and one oxygen atom, resulting in a consistent mass ratio of hydrogen to oxygen in every sample of water, regardless of where it is found or how it is produced. This unique composition defines the identity of the compound and differentiates it from others.

In contrast, other choices focus on different chemical principles. The Law of Multiple Proportions refers to how elements can combine in different ways to form different compounds, while Atomic Theory relates to the nature and behavior of atoms. Kinetic Molecular Theory deals with the behavior of gases in terms of particle motion. Each of these laws and theories has its own significance within chemistry but does not directly address the uniqueness of a compound's composition as clearly as the Law of Definite Composition does.