Which of the following statements accurately describes atomic mass unit?

The atomic mass unit (amu) is defined as a unit of mass that quantifies the mass of an atom relative to a standard reference, specifically the carbon-12 isotope. This definition is crucial because it allows scientists to compare the masses of different atoms on a consistent scale. The choice accurately states that an atomic mass unit expresses the mass of an atom relative to a carbon-12 atom, which is assigned a mass of exactly 12 amu. This relativity is essential in chemistry and physics, as it enables the determination of molecular weights and stoichiometric calculations based on the masses of various elements.

In contrast, the other options do not accurately represent the atomic mass unit. For instance, measuring speed pertains to velocity rather than mass, suggesting a misunderstanding of what atomic mass units quantify. Describing atomic mass as defining the energy of an atom also is misleading, as atomic mass focuses solely on mass, not energy levels or states. Furthermore, stating that atomic mass units are specific only to radioactive atoms is incorrect, as amu applies to all types of atoms, not just those that are radioactive.