Which statement concerning GMDSS maintenance requirements is true?

The statement about GMDSS maintenance requirements that is true is that the options include duplication of equipment, at-sea maintenance, and shore-based maintenance. This reflects the comprehensive nature of maintenance requirements under the Global Maritime Distress and Safety System (GMDSS).

The idea of duplication of equipment is critical in ensuring redundancy in communication systems, which enhances safety during distress situations. At-sea maintenance is essential as it allows for immediate repairs and troubleshooting without needing to return to port, while shore-based maintenance ensures that equipment is inspected and serviced regularly when in port to keep systems functional and compliant with regulations.

This comprehensive approach to maintenance helps ensure that vessels are always equipped to handle emergencies, highlighting the importance of maintaining communication equipment in good working order.