Recall the three main subatomic particles: electron, proton, and neutron, and understand that an alpha particle is a cluster of two protons and two neutrons.
Identify the relative masses of each particle: electrons have a much smaller mass compared to protons and neutrons.
Note that protons and neutrons have masses close to 1 atomic mass unit (amu), while electrons have a mass approximately 1/1836 of a proton.
Understand that an alpha particle, being composed of protons and neutrons, has a mass roughly four times that of a proton.
Conclude that among the given options, the electron has the lightest mass.