The mass number of potassium is:

Correct option is A

39
 Potassium is a chemical element with the symbol K and atomic number 19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to form flaky white potassium peroxide in only seconds of exposure.
 The element potassium has an atomic number 19 and a mass number is 39.