What legal document initiates a lawsuit?

The correct choice is the document that formally sets the legal process in motion by outlining the plaintiff's claims against the defendant. In legal terms, this is known as a complaint. The complaint serves several purposes: it notifies the defendant of the nature of the allegations being made against them, provides a basis for the court's jurisdiction, and outlines the particulars of the case, including the grounds for the lawsuit and the relief being sought.

While the summons also plays an important role in the initiation of a lawsuit by notifying the defendant that a legal action has been commenced against them, it is the complaint that actually contains the substantive details of the claims. The answer and motions are procedural documents used at different stages of legal proceedings following the initiation of a lawsuit, and they do not initiate a lawsuit themselves. Thus, the complaint is the foundational document that represents the beginning of legal action in the court system.