The maximum length of a power-driven vessel which may show an all-round white light and sidelights instead of a masthead light, sidelights and a stern light is:

The maximum length of a power-driven vessel that may display an all-round white light and sidelights instead of the more complex arrangement of masthead light, sidelights, and stern light is indeed 12 meters, which aligns with the standard for vessels under the navigation rules. While the question mentions several choices regarding lengths, the critical detail is that vessels under 12 meters are allowed to show a simplified light configuration, making it easier for smaller vessels to comply with navigation light requirements.

This regulation aims to enhance safety on the water by providing a clear visual signal to other vessels while accommodating the operational simplicity for smaller boats. Vessels over this length would need to adhere to more traditional and comprehensive lighting requirements, ensuring visibility and signaling capabilities align with their size and operational standards. Understanding these specifications is crucial for vessel operators to ensure compliance with navigation rules while promoting safety at sea.