Which document formally initiates a lawsuit?

The correct answer is the complaint because it is the document that formally initiates a lawsuit by outlining the plaintiff's claims against the defendant. A complaint sets forth the factual background of the case, the legal basis for the plaintiff's claims, and the specific relief sought from the court. It is typically filed with the court and then served to the defendant, giving the defendant notice of the lawsuit and the opportunity to respond.

In contrast, while the summons is also part of the initiation process, it is essentially a document that notifies the defendant that they have been sued and instructs them on how to respond to the complaint. The motion refers to requests made to the court for a specific ruling or order during the course of litigation, rather than initiating the case itself. An affidavit is a sworn statement of fact, often used as evidence in court, but it does not serve the purpose of starting a lawsuit.