What is the definition of an atomic mass unit (AMU)?

• A. 1 AMU is equal to the mass of a proton.
• B. 1 AMU is equal to the mass of a hydrogen atom.
• C. 1 AMU is equal to the mass of an electron.
• D. 1 AMU is equal to 1/12th the mass of a carbon atom.

Answer: (D)