Correct Option
An alpha particle is the nucleus of a helium-4 atom. It consists of two protons and two neutrons. Due to the absence of electrons, it carries a net charge of +2. Its mass is therefore approximately equal to the mass of a helium atom, as the mass contributed by electrons in a neutral helium atom is negligible compared to that of the nucleus. Thus, the mass of an alpha particle is very nearly equal to that of an atom of helium.
Incorrect Options
(a) An alpha particle comprises two protons and two neutrons. Therefore, considering only two protons would significantly underestimate its total mass, as the mass of two neutrons is omitted.
(c) An alpha particle is composed of nucleons (protons and neutrons). Positrons are antiparticles of electrons and are not constituents of an alpha particle's nucleus. Its mass is derived from its two protons and two neutrons, not from positrons.
(d) While a positron carries a single positive charge, its mass is negligible compared to that of a proton or neutron. The question specifically asks about mass equivalence, not just charge equivalence. An alpha particle's mass is primarily due to its four nucleons.