Final answer:
The particle with the least mass is the beta particle, which is a high-speed electron with a negligible mass compared to protons, neutrons, and alpha particles.(Option 2)
Step-by-step explanation:
The particle with the least mass among the options given is the beta particle, which is essentially a high-speed electron. These beta particles have a mass that is negligible compared to that of protons, neutrons, and alpha particles.
To clarify, an electron (the constituent of a beta particle) has a mass of approximately 0.00055 atomic mass units (amu), which is significantly less than that of a proton (1.0073 amu), neutron (1.0087 amu), or an alpha particle (which as a helium nucleus, has a mass of around 4 amu). The beta particle has such a small mass that about 1800 electrons would be required to equal the mass of just one proton. Based on this information, the correct answer to which particle has the least mass is option (2) beta particle.