Which of the following lights would be used to warn pilots of hazardous obstructions?

Red lights are specifically used to warn pilots of hazardous obstructions. The aviation color coding system designates red lights for warning or alert purposes, making them readily recognizable to pilots navigating around airports, particularly during low visibility conditions or at night. These lights help in identifying physical obstacles that may pose a risk to aircraft during takeoff and landing.

Other colors have different meanings; for instance, green lights are typically associated with runway thresholds or safe conditions, white lights are generally used in various navigational aids, and blue lights are often utilized for taxiway edge markings. As such, the use of red lights as a warning indicator plays a crucial role in enhancing situational awareness and safety for pilots operating in proximity to hazardous areas.