Which document would be used to announce public legal regulations?

The federal register is the correct document used to announce public legal regulations. It serves as the official journal of the federal government of the United States, where new regulations, proposed rules, and public notices are published. This allows for transparency and public awareness regarding changes in law and regulation, ensuring that individuals and entities are informed about the regulatory landscape that may affect them.

In contrast, a court order is a directive issued by a judge in a specific legal case, which does not serve the purpose of announcing regulations. A legal brief is a document presented to a court outlining the arguments for a case, focusing on legal reasoning rather than public regulation announcements. An attorney's letter typically communicates on behalf of a client or gives legal advice, and does not serve as an official publication for regulations. Thus, the federal register is uniquely suited for the task of making legal regulations publicly known.