The best standard for the calculation of relative atomic masses is:

Explanation

The best standard for the calculation of relative atomic masses is based on the International Union of Pure and Applied Chemistry (IUPAC) recommendation. The current standard is based on the carbon-12 isotope. The relative atomic mass of an element is determined by comparing the mass of its atoms to the mass of carbon-12 atoms. Carbon-12 is defined as having a relative atomic mass of exactly 12 atomic mass units (amu). This means that one atom of carbon-12 is assigned a mass of exactly 12 amu.Using carbon-12 as the reference, the relative atomic mass of other elements is calculated by comparing the masses of their atoms to the mass of a carbon-12 atom. This is done by measuring the mass of isotopes of the element and their relative abundance in nature. Isotopes are atoms of the same element that have different numbers of neutrons in their nuclei and, therefore, different masses.The atomic masses of elements are expressed relative to the carbon-12 scale using a unit called the atomic mass unit (amu). One atomic mass unit is defined as 1/12th of the mass of a carbon-12 atom.It's important to note that the relative atomic masses of some elements are not whole numbers because they are calculated based on the average masses of the isotopes present in nature and their relative abundances.The IUPAC periodically reviews and updates the atomic masses of elements based on new experimental data and advances in measurement techniques. This ensures that the atomic masses remain accurate and consistent over time.Atomic mass is defined as the mass of an atom of an element compared with the mass of an atom of carbon 12 (the stable light isotope of carbon). Atomic mass is a ratio and therefore has no units. It may be given any unit of measure, but generally, atomic masses are expressed in atomic mass units (a.m.u). One atomic mass unit equals exactly 1/12th the mass of a carbon 12 atom.Therefore, the most common standard for measuring an atom is the mass of carbon-12 and hence, option B is the correct answer.