Which of the following signals can indicate danger to others on site?

Waving both arms is a universally recognized signal that can indicate danger or the need for immediate attention on a construction site. This type of signal is typically used in situations where there is a potential hazard or when machinery may be in motion, as it can easily attract the attention of nearby personnel and communicate urgency. The visible and exaggerated movement of both arms helps to ensure that the signal is noticed from a distance, making it an effective way to alert others to a dangerous situation.

Other signals, such as a thumbs up or a point toward the sky, generally convey positive communication or directions rather than warnings. A clear hand signal might provide specific instructions but may not necessarily convey an urgent warning about danger. Thus, the waving of both arms stands out as a clear, instinctive alert signal that can effectively communicate potential danger to those on site.