Which of the following may display a white light only?

The choice indicating that a white light only may be displayed for safe water is correct because safe water marks are designed specifically to indicate that the water is deep enough for safe navigation and does not have any obstructions. These marks typically use a white light to signify that vessels can freely navigate in these areas without the danger of running aground.

Safe water marks can be identified not just visually through their unique colors and shapes, but also through their lights, which are intended to provide clear guidance to mariners during both daylight and nighttime conditions. The white light is a standard for safe water marks, which helps sailors ensure they are following safe navigational routes.

In contrast, restricted water and danger zone marks are associated with navigational risks and may use other signaling systems to warn vessels, such as red lights or additional signals indicating caution. Anchorage areas may also use various light signals to denote their purpose and ensure safety for anchoring, but are not restricted to displaying only white lights.