Question

Why carbon-12 isotope taken as the element?

Answer 1

Carbon -12 is taken as element because it is most stable among other isotopes

Answer 2

Though the ratio of any two atom's masses was the same on either scale, it was horribly confusing, so in 1961, a compromise was reached. Instead of using either Hydrogen, or Oxygen as the standard, the isotope of Carbon with 6 protons and 6 neutrons in its nucleus (Carbon-12) was given a mass of exactly 12.