b, bc the atomic mass is 12 and carbon has 6 protons (atomic number=# of protons).
to get electrons, u subtract atomic number (# of protons) from atomic mass, which gives u 6 electrons.
with 6 protons and 6 electrons, they are balanced.
not like a, which will have 6 protons but 5 electrons, meaning Carbon-11 will look for one more electron to balance things.