Which unit is used to express the mass of an atom, defined as one-twelfth the mass of a carbon-12 atom?

The appropriate unit used to express the mass of an atom, defined as one-twelfth the mass of a carbon-12 atom, is the atomic mass unit, often abbreviated as "amu." This unit is specifically designed to provide a scale for measuring atomic and molecular masses. By convention, the carbon-12 isotope is utilized as a standard reference point, making it essential to understand the relationship between this standard and the atomic masses of all other elements.

Choosing atomic mass units allows scientists and chemists to compare and relate the mass of different atoms and molecules conveniently and consistently. This unit aids in calculations involving chemical reactions and molecular weights, which are fundamental components in both chemistry and physics.

The other options do not accurately describe the unit of measurement for atomic mass. The atomic number refers to the number of protons in an atom's nucleus, while the mass number is the total count of protons and neutrons in an atom. The term "standard mass unit" is not a recognized terminology in the context of atomic mass; hence it does not apply here.