Which of the following is a form of nonionizing radiation?

Nonionizing radiation refers to types of radiation that do not carry enough energy to ionize atoms or molecules, which means they cannot remove tightly bound electrons. Ultraviolet radiation falls into this category because it has lower energy than ionizing radiation such as X-rays and gamma rays.

While ultraviolet radiation has the potential to cause damage to biological tissues (for example, leading to sunburn), it does not have sufficient energy to break chemical bonds or ionize atoms, which is a key characteristic that distinguishes nonionizing from ionizing radiation. X-rays and gamma rays, on the other hand, are both types of ionizing radiation, capable of causing ionization and resulting in significant biological effects. Alpha radiation, despite being a form of particle radiation, is also considered ionizing radiation due to its ability to cause ionization upon interacting with matter.

In summary, ultraviolet radiation is correctly identified as a form of nonionizing radiation due to its inability to ionize atoms, differentiating it clearly from other forms of radiation listed in the question.