what meant by 12c

Carbon-12 is the more abundant of the two stable isotopes of the element carbon, accounting for 98.89% of carbon; it contains six protons, so neutrons and six e- . Its abundance is due to the triple alpha process by which it is created in stars.

Carbon-12 is of particular importance as it is used as the standard from which atomic masses of all nuclides are measured: its mass number is 12 by definition.

hope it helps

Inder