Which light must a 20-meter vessel display when anchoring at night?

When a 20-meter vessel is anchoring at night, the regulations specify that it must exhibit an all-round white light to indicate its presence to other vessels. This all-round white light serves as a critical navigational aid, clearly signaling that the vessel is at anchor and is not underway. It helps other mariners to recognize the vessel's position in the dark, thus ensuring safety and preventing collisions.

In terms of other light configurations: the red and green sidelights are used when a vessel is underway; they indicate the vessel's port (left) and starboard (right) sides, which is not applicable when the vessel is at anchor. A flashing white light is typically used in specific scenarios, such as for certain types of vessels engaged in special operations or in distress signals, but it is not a requirement for an anchored vessel. Two all-round red lights indicate a vessel that is restricted in its ability to maneuver, which also doesn't apply to a vessel simply at anchor.

Thus, the requirement for an anchored vessel to display one all-round white light ensures that it is clearly visible to other vessels in the vicinity, facilitating safe navigation in the night.