If a vessel displays three all-round red lights in a vertical line at night, during the day she may show __________.

The correct choice is to show a cylinder during the day, which corresponds to the night display of three all-round red lights. The configuration of three all-round red lights indicates that the vessel is constrained by its draft. According to the International Regulations for Preventing Collisions at Sea (COLREGs), a vessel that has to display such lights at night must also show a specific day signal.

In this case, the appropriate day signal for a vessel constrained by its draft is a cylinder. This is clearly specified in Rule 30 of the COLREGs, which outlines the day shapes for specific vessel situations. The display of a cylinder is a recognized signal that informs other vessels of the vessel’s constrained condition during daylight.

Other options either do not correspond to the night signal or are not appropriate for the vessel's situation. An anchor light is not relevant as it indicates that the vessel is at anchor, not constrained by its draft. Two white lights do not provide sufficient information regarding the vessel's restricted condition, and having no signal at all would not communicate the vessel's status effectively, leading to potential navigation hazards. Thus, showing a cylinder is the correct and safe practice for this scenario.