A.helium<span>Hydrogen is the most abundant element in the Universe; helium is second. However, after this, the rank of abundance does not continue to correspond to the atomic number; oxygen has abundance rank 3, but atomic number 8.</span>
The most abundant of all of the isotopes of an element will be the one who's mass the mass of element is closest to. In this case, the mass of atomic carbon is closest to the mass of carbon-12.
Thus, Carbon-12 is the most abundant isotope.
This process is called meiosis! good luck!