If a radioactive element has a half-life less than or equal to 120 days, then it can be disposed of by __________.

The correct answer is based on the disposal guidelines for radioactive waste with a short half-life. If a radioactive element has a half-life of 120 days or less, it is generally possible to dispose of it by storing the material until it decays to a safe level. This method is often utilized because these materials lose their radioactivity relatively quickly compared to longer-lived isotopes.

Storing and allowing the radioactive material to decay in a controlled environment ensures that it loses its hazardous properties before being handled further or disposed of. This approach minimizes risks to health and the environment, as the radioactivity decreases over time.

In contrast, chemical neutralization, burial, and incineration may not be suitable for all types of radioactive materials, especially those with longer half-lives, as these methods might not sufficiently mitigate the risks associated with radioactive waste, and may even lead to further environmental concerns. Each method has specific guidelines and regulations that dictate their use based on the type and longevity of the radioactive materials in question.