The correct answer is option 3 - The number of electrons in argon is 18.

To understand why, let`s break down the mass number of argon (40). The mass number of an element is the sum of the number of protons and neutrons in its nucleus. Therefore, since the mass number of argon is 40, it means that there are 40 particles in its nucleus, including protons and neutrons.

Now, let`s move on to the number of protons in argon. The atomic number of an element is the number of protons in its nucleus. Since argon`s mass number is 40, it means that it has 40 - 18 = 22 neutrons.

Next, the number of electrons in an atom is equal to the number of protons. This is because atoms are electrically neutral, meaning they have an equal number of positive protons and negative electrons, balancing out their charge. Therefore, the number of electrons in argon is also 18.

Lastly, option 4 states that the sum of the numbers of protons and electrons in argon is 40. This is not correct. As discussed, argon has 18 protons and 18 electrons, making the