The types of radiation emitted by a radioactive substance are:

Radioactive emissions are commonly classified into three types — alpha (α), beta (β) and gamma (γ). A given radioactive substance, as it decays, can give off any of these, so all three are recognised types of radiation emitted by radioactive substances.

Alpha rays (α): positively charged helium nuclei (2 protons + 2 neutrons). They are heavy, have the strongest ionising power, but the least penetrating power (stopped by a sheet of paper).

Beta rays (β): fast-moving electrons (β⁻, emitted when a neutron converts into a proton) or, in β⁺ decay, positrons (emitted when a proton converts into a neutron). They are lighter and more penetrating than alpha rays (stopped by a few millimetres of aluminium).

Gamma rays (γ): high-energy electromagnetic radiation (photons) with no charge or mass. They have the highest penetrating power (need thick lead or concrete to be stopped) but the weakest ionising power.

Since alpha, beta and gamma are all types of radiation emitted by radioactive substances, the correct answer is “All of these.”