Final answer:
The correct response is that assertion A is true, as one atomic mass unit is defined as one twelfth of the mass of one carbon-12 atom, but reason R is false since carbon-12's standard selection wasn't solely due to abundance.
Step-by-step explanation:
The correct answer to the question is D. A is true but R is false. The assertion (A) is indeed true; one atomic mass unit (amu or u) is defined as one twelfth of the mass of one carbon-12 atom. The carbon-12 atom is made up of six protons and six neutrons in its nucleus, giving it a total of twelve nucleons. By this standard, a proton has a mass of approximately 1.00728 u, a neutron has a mass of 1.00866 u, and the mass of an electron is about 0.000549 u.
However, the reason (R) given is false. While carbon-12 is indeed an isotope of carbon, its selection as the standard for defining the atomic mass unit was not because it is the most abundant isotope. It was chosen for reasons including its neutrality (having equal numbers of protons and neutrons) and its importance in organic chemistry. Furthermore, its atomic mass aligning neatly with the mass number (12 amu) provides a convenient basis for atomic mass calculations.