Explanation:
A trigger is a warning sign or a condition that a risk event is likely to occur within the project.
Incorrect Answers:
A: Issues are events that come about as a result of risk events. Risks become issues only after they have actually occurred.
B: A contingency response is a pre-planned response for a risk event, such as a rollback plan.
D: A threshold is a limit that the risk passes to actually become an issue in the project.