Which word could describe an event that is unavoidable?

The term that best describes an event that is unavoidable is "inevitable." This word implies that no matter the circumstances or actions taken, the occurrence of the event is guaranteed. It conveys a strong sense that the event will happen without fail, and there is no way to prevent it from taking place.

In contrast, "contingent" suggests that the occurrence of an event depends on certain conditions or factors. "Preventable" clearly indicates that an event can be stopped or avoided, which is the opposite of being unavoidable. Meanwhile, "certain" denotes a strong likelihood or guarantee of occurrence, but it does not carry the same definitive sense of inevitability as "inevitable." Therefore, "inevitable" is the most appropriate term to use when describing something that cannot be avoided.