What is the difference between relative atomic mass and Atomic mass unit

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Atomic weight, also referred to as relative atomic mass, is the ratio of the mean mass of the atoms of a chemical element to a certain standard. The normal unit of atomic mass has been one-twelfth of the atomic mass of the carbon-12 isotope since the year 1961.

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.