What formal document indicates that a lawsuit is being filed against an insured?

The correct choice is a formal document known as a "Suit and summons." This document serves a critical role in legal proceedings by formally notifying the defendant (in this case, the insured) that a lawsuit has been initiated against them. It outlines the claims being made and specifies the time frame within which the defendant must respond to the complaint.

When a lawsuit is filed, the suit and summons are typically delivered to the insured to ensure that they are aware of the legal action and can prepare a defense. This is vital in the legal process as it upholds the principle of due process, ensuring that individuals have an opportunity to respond to allegations made against them.

Other options do not fulfill this purpose: a claim adjustment typically relates to the processing of insurance claims rather than the initiation of a lawsuit; a policy endorsement modifies the terms of an insurance policy itself; and a legal brief is a document presented to the court that argues a party's legal position, but it does not serve as formal notice of a lawsuit being filed. Therefore, the suit and summons is the specific document that denotes the commencement of legal action against an insured.