Answer:
D. one-twelfth the mass of one carbon-12 atom
Explanation:
A is false because an atom is far too small to be compared to an atomic mass unit. Actually often its mass is considered negligible
B. A helium atom has a mass of four atomic mass units not one
C. The carbon-12 atom has a mass of 12 atomic mass units, as its name suggests
D. One atomic mass unit is equal to a twelfth of the mass of a carbon 12 atom. It is actually the definition of an atomic mass unit. It was once based on the hydrogen atom but today scientists define it using the carbon 12 atom
