Final answer:
Relative masses of atoms and subatomic particles are measured in atomic mass units (amu), which is defined as one-twelfth the mass of a carbon-12 atom, making the statement True.
Step-by-step explanation:
Relative masses are indeed measured in units of atomic mass units (amu). This is because atoms and subatomic particles are exceptionally tiny, making it impractical to use grams for their mass. Instead, the atomic mass unit is used, which is defined as one-twelfth of the mass of a carbon-12 atom. This atom consists of 6 protons, 6 neutrons, and 6 electrons. Therefore, the statement is True. The atomic mass of an element allows us to compare the mass of different elements relative to this standard. For instance, helium-4 has an atomic mass of 4.0026 amu, which is relative to carbon-12's defined mass of 12 amu.