An alpha particle is composed of two protons and two neutrons, which are the same components as a helium atom. Thus, their mass is essentially the same.

Option 1 suggests that the alpha particle is equivalent to the mass of two protons. But this isn`t correct as an alpha particle also includes two neutrons in addition to two protons.

Option 3 refers to the mass of two positrons and two neutrons. Positrons carry the same amount of positive charge as protons, but they are the anti-particles of electrons, so their mass is much smaller than either protons or neutrons, making this option incorrect.

Option 4 says that the alpha particle`s mass is equal to that of two positrons. As mentioned before, positrons have much smaller mass than protons or neutrons, making this option incorrect as well.

Thus, option 2, "an atom of helium" is the correct answer, as an alpha particle’s mass is very similar to that of a helium atom, due to their identical compositions.