To determine the mass number of an atom, you need to add the number of protons and neutrons together. Potassium (K) has an atomic number of 19, which means it has 19 protons. If this particular potassium atom has 20 neutrons, you would calculate the mass number as follows:
Mass Number = Number of Protons + Number of Neutrons
Mass Number = 19 (protons) + 20 (neutrons) = 39
Therefore, the mass number of the potassium atom in question is 39. The correct option is d) 39.