Question

Why ${}^{12}\text{C}$ is the basis of the modern atomic mass unit?

Answer 1

${}^{12}\text{C}$ is the basis of atomic mass scale.
The atomic mass of ${}^{12}\text{C}$ is 12.000 atomic mass units abbrevated as 12.000 amu. 1 amu is 1/12th of the mass of an atom of carbon -12 equal to $1.6605\times {19}^{-24}g$