The relative atomic masses of all elements have been found with respect to an atom of ________.

The International Union of Pure and Applied Chemistry (IUPAC) chose Carbon-12 (C-12) as the standard for measuring atomic masses. The mass of a C-12 atom is defined as exactly 12 atomic mass units (amu). This isotope contains six protons and six neutrons, providing a reliable reference point for determining the atomic masses of other elements.