The correct answer is option-3: Red light is scattered the least.

When it comes to danger signals, red light is employed because it is scattered the least compared to other colors. This means that red light can travel through longer distances without losing its intensity and visibility.

Option-1, which states that red color is soothing to the eye, is not a valid reason for employing red light for danger signals. The purpose of a danger signal is to grab attention and alert individuals to potential hazards, so using a soothing color would not be effective.

Option-2, stating that the human eye is most sensitive to red color, is not the primary reason for using red light for danger signals. While it is true that the human eye is more sensitive to red light compared to some other colors, the main reason for using red light is its low scattering property.

Option-4, which states that red light is scattered the most, is incorrect. If red light were scattered the most, it would lose its intensity and visibility over shorter distances, making it less effective for danger signals.

In summary, red light is employed for danger signals because it is scattered the least, allowing it to travel long distances without losing its visibility.