Which document formally initiates a legal proceeding against an individual?

The document that formally initiates a legal proceeding against an individual is a complaint. A complaint sets forth the facts of the case, the legal grounds for the lawsuit, and the specific relief sought by the plaintiff. It is the first step in a lawsuit and provides the court with the necessary information to understand the nature of the dispute.

While a summons does notify the defendant that a legal action has been initiated against them and provides them the details on how to respond, it is the complaint that contains the substance of the plaintiff's claims and formally begins the legal process in court. Therefore, the complaint is the correct answer here, as it is the essential document that outlines the basis of the legal action.