The atomic mass of an element is defined as the sum of the number of protons and neutrons in an atom of that element.

Option 1 states that the atomic mass is equal to the sum of the number of electrons and protons only. This is incorrect because the atomic mass does not include the number of electrons.

Option 2 states that the atomic mass is equal to the sum of the number of protons and neutrons only. This is the correct answer. The atomic mass is determined by adding together the masses of protons and neutrons, as electrons have a negligible mass in comparison.

Option 3 states that the atomic mass is equal to the sum of the number of electrons and neutrons only. This is incorrect because, as mentioned earlier, the atomic mass does not include the number of electrons.

Option 4 states that the atomic mass is equal to the sum of the number of electrons, protons, and neutrons. This is incorrect because, as explained previously, the atomic mass does not include the number of electrons.

In summary, the correct answer is option 2, where the atomic mass is equal to the sum of the number of protons and neutrons only.