An active shooter is best defined as:

The definition of an active shooter primarily revolves around an individual who is actively engaged in killing or attempting to kill people in a populated area, typically using firearms. This scenario emphasizes not just the presence of a weapon, but the willingness and intent to use it against others to cause harm.

The correct choice highlights the crucial aspect of active shooter situations: the use of deadly force against unsuspecting victims. This definition is essential for law enforcement and emergency response teams as it sets the parameters for their immediate response and action plans when such an incident occurs.

In contrast, while the other definitions may relate to violent behavior or potential threats, they do not encapsulate the specific and immediate threat posed by someone actively seeking to harm individuals in a community setting. For instance, participating in violent demonstrations, experiencing extreme mental distress, or possessing violent content do not necessarily indicate that an individual is engaged in an active shooting event. Understanding this definition is vital for effective training and preparedness in responding to such critical incidents.