Which of the following indicates a radiological/nuclear WMD event may have occurred?

The presence of an unexplained explosion or bomb-like material serves as a significant indicator that a radiological or nuclear Weapon of Mass Destruction (WMD) event may have taken place. Exploring this option helps clarify the associated risks: when a radiological or nuclear device detonates, it is often accompanied by a loud explosion, which can lead to immediate concerns about radiation exposure and contamination. This could provoke alarms due to the possibility of significant casualties and the spread of radioactive materials, necessitating urgent emergency response.

In contrast, while unusual mail, power outages, or public disturbances may suggest other types of emergencies or incidents, they do not specifically point toward a WMD event involving radiological or nuclear components. Unusual mail could indicate other threats, such as biological or chemical agents. Power outages might arise from various causes unrelated to WMD, such as weather conditions or technical failures. Public disturbances can result from civil unrest or other societal issues but lack a direct link to radiological concerns unless tied specifically to a WMD context. Therefore, the explosion or bomb-like material is the most telling sign of a potential nuclear or radiological incident, emphasizing the need for immediate investigation and response by emergency services.