A survey (special purpose mark) buoy must be?

A special purpose mark, such as a survey buoy, serves a unique role in supporting specific activities like scientific research or data collection and is not primarily intended for navigation. Therefore, it does not require lighting similar to lighthouse beacons or navigational aids that assist mariners in safely charting their course.

This buoy type is often unlighted, making it less conspicuous but still adequate for its intended use. In contrast to navigational markers, which are designed to communicate immediate warnings or guide vessels, survey buoys operate with a different set of standards and purposes that do not necessitate illumination. Thus, the answer indicating that a survey buoy can be "none of the above" accurately reflects the guidelines for these aids to navigation, aligning with their specific function and regulatory requirements.