Final answer:
An alpha particle, which consists of two protons and two neutrons, is equivalent to a helium atom nucleus and is the correct answer to the student's question. During the process known as alpha decay, a radioactive atom emits an alpha particle, which results in the atom's atomic number decreasing by two and its mass number by four.
Step-by-step explanation:
The form of radiation that is equivalent to a helium atom nucleus is an alpha particle. An alpha particle is composed of two protons and two neutrons, which is the same configuration as a helium nucleus often represented as He or Helium-4. During the process known as alpha decay, a radioactive atom emits an alpha particle, which results in the atom's atomic number decreasing by two and its mass number by four, due to the loss of these nuclear particles.