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An alpha particle is defined as a cluster of 2 protons and 2 neutrons, which is equivalent to the nucleus of a helium atom. This composition gives alpha particles a plus two charge due to the presence of the two protons, and they are relatively heavy compared to other types of radiation, such as beta particles or gamma rays.
In radioactive decay, alpha particles are emitted by certain types of heavy nuclei, and their mass and charge contribute to their ability to ionize atoms in their path. They have low penetration power and can be stopped by a sheet of paper or human skin, which is an important factor in understanding radiation shielding and safety measures.
The other choices do not accurately describe the nature of alpha particles. A high-speed electron refers to a beta particle, which has different properties. A photon is a quantum of electromagnetic radiation with no mass and is distinct from alpha particles in that it carries energy without charge. A neutron is a different subatomic particle and is not emitted in the same manner as alpha particles but is involved in other types of nuclear reactions. Thus, the characterization of an alpha particle as a cluster of 2 protons and 2 neutrons is fundamentally accurate.