Final answer:
The warning stage that would be initiated if a terrorist event were likely to occur within 48 hours is Imminent.
Step-by-step explanation:
The warning stage that would be initiated if a terrorist event were likely to occur within 48 hours is Imminent.
In the given context, the term 'imminent' refers to a situation where there is a clear and immediate risk of an event taking place, such as a terrorist attack.
According to the provided information, the stages of warning for a potential hazard are ranked from 0 to 4, with 4 indicating the highest level of danger. The stage 'Imminent' signifies that the event is highly likely to happen soon, indicating extreme physical danger.