The atomic mass of most common isotope of carbon in ground state is:

The correct option is A

equal to 12 u

The most common isotope of carbon which contains six protons, six neutrons and six electrons is carbon-12 (C-12).

The unified atomic mass unit, 'amu', is defined as $(\frac{1}{12}{)}^{th}$ of the mass of a neutral carbon-12 in its lowest energy state which contains six protons, six neutrons and six electrons.
So, by definition, a carbon-12 atom in ground state has a mass of exactly 12 amu.