The atomic mass of an element is defined as the weighted average mass of that element's? (1) most abundant isotope (3) naturally occurring isotopes (4) radioactive isotopes

I think the correct answer from the choices listed above is option 3. The atomic mass of an element is defined as the weighted average mass of that element's naturally occurring isotopes. The atomic mass of a single atom is simply its total mass and is typically expressed in atomic mass units or amu.