Which of the following is not a type of ionizing radiation?

Ionizing radiation refers to particles or waves that have enough energy to remove tightly bound electrons from atoms, thus creating ions. The types of ionizing radiation include alpha particles, beta particles, and gamma rays.

Alpha radiation consists of helium nuclei and is relatively heavy, carrying a positive charge. Beta radiation is made up of high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei. Gamma rays are electromagnetic radiation with high frequency and energy, often accompanying other forms of decay.

The choice that does not belong to this group is cloud. Unlike the other three options, "cloud" does not refer to a type of ionizing radiation but may relate to a concept in a different context, such as meteorological phenomena or possibly the visual representations of radiation fields in certain models. Thus, recognizing that alpha, beta, and gamma are all categories of ionizing radiation clarifies why "cloud" is the correct answer to this question.