As a sailing vessel using an engine for propulsion, what day signal should be shown?

A sailing vessel that is using an engine for propulsion does not have a specific day signal that it is required to display to indicate its mode of operation. Instead, the absence of a designated day signal indicates that the vessel is operating under power.

Vessels under sail, especially when they are not using engine propulsion, may display a specific signal to indicate they are sailing, but this does not apply when the vessel is being propelled by an engine. Similarly, displaying flags such as a white flag or a red flag in this context would not be appropriate, as these signals have different meanings and uses in maritime signaling.

Therefore, the correct response that captures the absence of any mandated signal for this situation is that none of the options provided are applicable.