Which of the following is the atomic mass of an atom containing 31 protons 31 electrons and 37 neutrons?

To calculate the atomic mass, sum the number of protons and the number of neutrons inside of the nucleus. The electron’s size is miniscule in comparison to both the proton and the neutron and does not contribute to the mass of the atom. So, adding 31 protons with 37 neutrons results in an atomic mass of 68.