The modern atomic mass unit if based on the mass of ………

Answer: C-12 isotopeSolution: Carbon-12 isotope. According to modern atomic mass unit, one atomic mass unit (amu) is defined as one-twelfth of mass of an atom of $C-12$ isotope, i.e.
1 amu $\left(u\right)=\frac{1}{12}\times \text{weight of an atom of C-12 isotope}$.