Which light configuration indicates that a sailing vessel is operating machinery?

The correct configuration indicating that a sailing vessel is operating machinery is represented by the side and masthead lights. When a sailing vessel, which is typically under sail, is engaged in mechanical propulsion, it must show these specific lights to signal its status to other vessels.

The side lights (green on the starboard side and red on the port side) and a masthead light (white light placed on the centerline of the vessel at least 2.5 meters above sidelights) clearly convey that the vessel is currently under power, transitioning its operational mode from sailing to using machinery. This lighting configuration is essential for safe navigation and to ensure that other boaters can identify its movement and intentions on the water.

In contrast, other configurations such as all-round lights or just a stern light do not adequately convey the operational status of a vessel equipped with machinery, which is why they are not the correct choices in this case.