What formal document compels an individual to appear in court to defend against a lawsuit?

The correct choice is a summons. A summons is a legal document that notifies an individual that they are being sued and compels them to appear in court to respond to the lawsuit. It is typically issued by the court and served to the defendant, outlining the nature of the action and providing a timeframe within which the individual must respond or appear.

In the context of legal proceedings, a summons serves as an essential procedural tool that ensures that the defendant is aware of the claims against them and has an opportunity to defend themselves. This document plays a crucial role in upholding the principles of due process, as it guarantees that individuals have notice of legal actions impacting their rights.

While a complaint initiates a lawsuit by outlining the plaintiff's claims, a subpoena is used to compel the attendance of witnesses or the production of evidence. An affidavit is a written statement confirmed by oath or affirmation, but it does not serve the purpose of requiring someone's appearance in court. Therefore, a summons is the formal document that directly addresses the requirement for an individual to appear in court in response to a lawsuit.