Final answer:
In the given sentence, 'expiration' refers to the end of a four-year period related to ongoing public declarations and discussions.
Step-by-step explanation:
The best definition of expiration as it is used in the sentence is the coming to an end of a period of time. In the context provided, expiration refers to the end of a four-year period during which public declarations have been made about a significant ongoing contest or event. In the medical or biological context, expiration can refer to the exhalation process when air leaves the lungs, but in the given sentence, it pertains to the passage of a certain amount of time.