Final answer:
The nucleus of a K-40 atom contains 19 protons and 21 neutrons, determined by subtracting the atomic number from the mass number.
Step-by-step explanation:
The nucleus of an atom of K-40 contains 19 protons and 21 neutrons. This can be determined by understanding that the atomic number of potassium (K) is 19, which means there are 19 protons in its nucleus. The mass number of K-40 represents the sum of protons and neutrons, which is 40. To find the number of neutrons, we subtract the number of protons from the mass number:
number of neutrons = 40 - 19 = 21.
Therefore, the correct answer is (a) 19 protons and 21 neutrons.