Which document would confirm the successful submission of a trademark-related fax?

The correct answer is the certificate of fax transmission, which serves as a confirmation that a fax has been successfully transmitted to the intended recipient. This document provides proof that the fax was completed without any errors or issues during the sending process, making it crucial for verifying the submission of trademark-related information via fax.

In contrast, a letter of protest pertains to a formal objection or challenge to a trademark application, generally submitted after the application is filed. A certificate of mailing, while useful for confirming that a document was sent through the postal service, does not apply in the context of faxes. Lastly, a trademark application is the initial document submitted for registering a mark and does not provide any confirmation regarding the method of submission, such as faxing.