The correct answer is C. According to U.S. Coast Guard regulations, a small passenger vessel's Official Number must be marked or mounted so its alteration, removal, or replacement would be obvious. This requirement ensures the vessel's identification can be easily verified and prevents tampering or obscuring of the official markings. The other answer choices, while partially correct, do not fully address the regulatory requirement for the Official Number to be visibly marked in a way that would make any unauthorized changes apparent. The number must not only be of a certain size (as in A) and located on the exterior of the vessel (as in B), but specifically mounted in a manner that would reveal any attempts to alter or remove it.