The unified atomic mass unit (u) is defined as exactly $1/12$ of the mass of an atom of which isotope?

1. NCERT §13.2 defines the unified atomic mass unit by international convention. 2. The reference standard is the neutral atom of carbon-12, $\,^{12}_{6}\mathrm{C}$, in its electronic ground state. 3. One unified atomic mass unit: $1\,u = \dfrac{1}{12}\,m(\,^{12}_{6}\mathrm{C}) = 1.66054\times 10^{-27}\,\text{kg}$. 4. Carbon-12 was chosen in 1961 because it gives integer-friendly mass values for most elements. 5. Options A, C, D were never the standard — though older systems (pre-1961) used $\,^{16}\mathrm{O}$ for chemistry (option C), this gave a slightly different unit than physics's $\,^{16}\mathrm{O}$ isotope-only definition. _Source: NCERT Class 12 Physics Part 2, Ch 13 "Nuclei", §13.2 (Atomic Masses), p. 1._