Atomic mass of carbon is:

The atomic mass of an element is the average mass of the atoms of the element measured in atomic mass units.
The atomic mass is the weighted average of all isotopes of that element, in which the mass of each isotope is multiplied by the amount of that particular isotope.
Atomic mass is equal to the number of protons and neutrons present in the element.
Now, the given element is carbon, represented by the symbol C. It belongs to Group 14 and its atomic number is 6. Thus, a neutral carbon atom has 6 neutrons and 6 protons in its nucleus.
So, 
$\text{Atomic mass = 6+6 = 12}$