What must a vessel under 7 meters and unable to exceed 7 knots primarily display at night?

A vessel under 7 meters in length that is unable to exceed a speed of 7 knots is primarily required to display an all-around white light at night. This requirement is outlined in the Navigation Rules, specifically designed to ensure the visibility of smaller vessels in the dark for safety and collision avoidance.

The all-around white light provides a single, continuous beam of light visible from every direction, which is crucial for informing other vessels of its presence and status. This is particularly important for navigating in areas with potential traffic or in waters where other vessels, including larger vessels, may not expect to encounter small crafts.

Additionally, the clarity of the all-around white light helps to minimize potential confusion for other mariners regarding the vessel's position and movement. In contrast, colored lights or multiple lights would not provide the same clarity for a vessel of this size and speed, which is why they are not the correct answer in this context. An anchor light is typically for vessels that are at anchor and does not apply for those that are moving or operating while underway.