Hint: The carbon atom had been used as a reference while assigning atomic mass to the elements of the periodic table.
According to the standard definition of the atomic unit of mass, it is defined as accurately of the mass of a carbon-12 atom. It is true Carbon-12 isotope is the most abundant isotope of carbon and has been chosen as the standard. The percentage of C-12 isotope is 98.93%.
but the correct reason - Carbon-12 is the standard while measuring the atomic masses. Because no other nuclides other than carbon-12 have exactly whole-number masses in this scale.
This is due to two factors:
[1] the different mass of neutrons and protons acting to change the total mass in nuclides with proton/neutron ratios other than the 1:1 ratio of carbon-12; and
[2] However, the primary reason carbon-12 was chosen is not its abundance but its stability, well-defined mass (6 protons and 6 neutrons), and the international agreement in 1961 by the International Union of Pure and Applied Chemistry (IUPAC) to use it as a universal reference to unify measurements in chemistry and physics.
Hence, both assertion and reason are true and the reason is not the correct explanation of assertion.
