Question

"Why 1/12" mass of carbon was chosen as the standard atomic mass for comparing atomic masses of other elements?

Answer 1

An atomic mass unit (symbolized AMU or amu) is defined as precisely1/12 the mass of an atom of carbon-12. The carbon-12 (C-12) atom has six protons and six neutrons in its nucleus. In imprecise terms, one AMU is the average of the proton rest mass and the neutron rest mass.

Answer 2

 the mass of an atom of carbon-12. The carbon-12  atom has six protons and six neutrons in its nucleus.