What is a brief form of correspondence used by addresses to approve, disapprove, or comment on the contents of a letter?

The correct choice identifies endorsement as the brief form of correspondence used to express approval, disapproval, or comments regarding the content of a letter. An endorsement typically follows the original document and provides the necessary feedback or decision from the reviewing individual or authority. This practice is common in formal communication within organizations, such as the Marine Corps, where clear and concise responses are essential for efficient processing.

While memos serve as internal communication tools that convey information or directives, they don't explicitly function as a response to the contents of another correspondence. Similarly, a letter of intent represents a formal agreement or understanding rather than a brief endorsement of a previous letter's contents. Notifications are generally used to inform recipients about events or changes and do not usually include a commentary or decision-making aspect regarding other documents. Thus, endorsement specifically fulfills the requirement of providing a succinct response to a letter, making it the most appropriate choice.