The correct answer is D) may be displayed during daylight. The navigation lights required for vessels must be displayed from sunset to sunrise, as stated in the question. However, these lights may also be displayed during daylight hours, if the vessel operator chooses to do so. This is because the navigation lights serve to make the vessel more visible to other vessels, which can be beneficial even in daylight conditions. The other answer options are incorrect because: A) navigation lights are not required when using day signals, B) navigation lights must be displayed when moored, not just when tied to a pier, and C) navigation lights cannot be extinguished at night on open waters, as they are required to be displayed from sunset to sunrise.