A radioactive substance emits
A. α-rays
B. β-rays
C. δ-rays
D. All of these

Hint: Radionuclides (or radioactive materials) are the elements in which the nucleus of the atom is unstable. They reach stability through variations in the nucleus (spontaneous fission, emission of alpha particles, or transformation of neutrons to protons or the reverse).

Complete step by step solution:
A radioactive substance is unstable and produces hazardous kinds of radiation. It is unstable because the strong nuclear force that holds the nucleus of the atom together is unbalanced with the electric force that needs to push it apart. Because it is unstable, the atoms will deteriorate into more stable ones.

Radioactive substances are continually producing three diverse kinds of dangerous radiation: alpha particles, beta particles, and gamma rays. These kinds of radiation are invisible to the naked eye, so people do not see a green glow. But sometimes they are able to interact with nearby fluorescent or phosphorescent materials that will glow green. Therefore, a green glow is linked with radioactivity.

Alpha particles are the nuclei of helium atoms. They move gradually, and don't penetrate the skin, but can cause a lot of harm if swallowed. Beta particles are high-energy electrons that can pass through paper and even part of the way into the human body but lead to less damage when they do. And gamma rays are incredibly high-energy electromagnetic waves that can only be stopped by dense lead or concrete.

Therefore the correct answer is option D.

Note: A radioactive atom has higher energy than it must have. Energy in the universe tends to stretch out, and so when something has more energy than it needs, it's only a matter of time as long as it loses that energy. When a radioactive atom loses its excess energy by decaying, that energy must go anywhere - energy in the universe is never generated or destroyed, it is said to be always preserved.