What must a defendant file in response to the plaintiff's demands?

In legal proceedings, when a plaintiff submits a complaint, the defendant is required to respond formally to the allegations presented. This response is known as an "Answer." An Answer allows the defendant to address each claim made by the plaintiff, including admitting or denying specific allegations and introducing any defenses or counterclaims that may be relevant to the case. It is an essential part of the litigation process as it helps to establish the issues that will be contested in court.

The other options represent different legal documents or concepts but do not fulfill the role of directly responding to the plaintiff's demands. For instance, an adverse report generally refers to documentation that indicates negative findings related to evidence or investigations, while a motion is a request made to the court for a specific ruling or order. An affidavit, on the other hand, is a sworn statement that provides evidence, but it does not function as a response to the plaintiff's allegations.

Therefore, the Answer is the correct document that a defendant must file to effectively engage with the plaintiff's demands in the litigation process.