Red light is used for danger signals because

Red light is used for danger signals primarily because it scatters the least (option A). Unlike shorter wavelengths such as blue or violet, red light tends to scatter less in the atmosphere. This property enables red light signals to be visible over longer distances, even in conditions of fog or haze where other colors might disperse more readily. Additionally, red light is perceived as less glaring and more comfortable for the eyes, which is beneficial in situations requiring prolonged attention to warning signals. Furthermore, red light has minimal chemical effects compared to other wavelengths, making it safer for both human observers and the environment. Its absorption by air is also relatively low, contributing to its effectiveness in maintaining signal visibility over distances. Therefore, the choice of red light for danger signals combines practical considerations of visibility, comfort, and safety, ensuring that warning signals are effective and reliable in alerting individuals to potential hazards or emergencies.