Relative atomic mass (Ar) is defined as which of the following?

Relative atomic mass (Ar) is defined as the average mass of an element's isotopes, weighted according to their natural abundance, relative to one-twelfth of the mass of a carbon-12 atom. This definition encompasses the contributions of all isotopes of an element, rather than focusing on just one specific isotope, whether it be the heaviest, the most stable, or any single isotope.

By considering the average and taking into account the relative abundance of each isotope, the relative atomic mass provides a more accurate representation of an element's mass as found in nature. This average is crucial in various applications in chemistry, such as stoichiometry, where precise mass values are needed to calculate moles and reactions.

The other options do not align with this definition because they either reference a single isotope, do not consider relative abundance, or express concepts that do not pertain to the standard definition of relative atomic mass.