What legal document serves as a notice of a lawsuit?

A summons is the correct answer because it is a legal document issued by a court that notifies a defendant that a lawsuit has been filed against them. The summons outlines the nature of the legal action, provides the details of the court where the lawsuit has been initiated, and specifies the time frame in which the defendant must respond to the complaint. This document is essential for ensuring that the defendant is aware of the legal proceedings and has an opportunity to defend themselves.

The other options serve different legal purposes. A contract is an agreement between parties that creates binding obligations but does not serve to notify someone of a lawsuit. An affidavit is a written statement confirmed by oath or affirmation, used as evidence in court, but it does not function as a formal notice of legal action. A subpoena, while also a legal document, is used to compel an individual to appear in court or produce evidence, rather than to inform someone about a lawsuit. Therefore, the summons is uniquely suited to the purpose of notifying a party about a lawsuit.