The mass number of potassium is :

39 40 38 37

39

The correct answer is option 1. 39. The mass number of potassium is 39. The mass number of an element represents the sum of its protons and neutrons. Potassium has 19 protons (its atomic number), so the remaining 20 particles in its nucleus must be neutrons, leading to a mass number of 39 (19 protons + 20 neutrons = 39).