Select the most suitable option from the given expression to complete the…

Idiom-based sentence-completion questions test whether the chosen phrase matches both the literal meaning implied by the context and the standard, fixed meaning of the idiom itself; an idiom cannot be swapped for a phrase that merely sounds plausible, its conventional sense must fit the situation described in the sentence.

The sentence describes guests staying at a gathering well past midnight and then deciding to stop; the missing phrase must express the action of ending or stopping an activity. Among the four options, only "to call it a day" carries the conventional meaning "to stop doing something for the time being", which is exactly the action the guests take when they decide the gathering is over. The idiom is used for ending any activity in general, not literally the daytime, so it still applies even though the scene is set well past midnight.

• "to land on his/her feet" means to recover successfully after a setback, which has no connection to guests ending a late-night gathering.

• "to face the music" means to accept criticism or consequences of one's own actions, an unrelated situation to guests wrapping up a gathering.

• "to end in a fiasco" describes an outcome turning into a failure, not an action that people consciously decide to take.

Substituting each candidate confirms that only "to call it a day" produces a sentence that is both grammatically natural after "decided" and semantically consistent with guests ending their evening, confirming "to call it a day" as the correct completion.