What signal should be shown during the day to indicate a bad or dangerous place to land?

The correct signal to indicate a bad or dangerous place to land during the day is the arms or white flag moving horizontally. This signal is recognized internationally as a warning for mariners, signifying that it is unsafe to approach or land in that area. The horizontal movement of a white flag indicates caution and alerts vessels to stay clear due to potential hazards, such as reefs, rocks, or other dangers present at the landing site.

In contrast, other signals provided in the options convey different meanings. Firing a white star signal or a green star signal typically indicates specific emergencies or navigational guidance, but they do not serve the purpose of warning against unsafe landing areas. The vertical motion of a red flag generally signifies a warning as well, but in most contexts, it is more commonly associated with the presence of danger without the explicit indication of being a bad place to land. Thus, the arms or white flag moving horizontally is the most appropriate and recognized signal for this specific situation.