Which lights are used to outline the structure forward of the helicopter?

The correct choice regarding the lights used to outline the structure forward of the helicopter is obstruction lights. These lights serve a specific purpose in aviation by marking obstacles or structures that could pose a hazard to aircraft, particularly when flying at lower altitudes or during takeoff and landing phases.

Obstruction lights are designed to be highly visible and often flash or remain steady, making them effective in indicating the presence of towers, buildings, or other structures that might obstruct air traffic. In a helicopter's operational context, outlining the structure clearly helps pilots navigate safely around and avoid these hazards.

Other types of lights, such as navigation lights, are primarily used to indicate the position and orientation of the aircraft during flight, while takeoff lights provide additional illumination when preparing for takeoff. Warning lights typically signal alert conditions or emergencies rather than specifically outlining structures. Thus, obstruction lights fulfill the requirement of marking the presence of an object effectively and enhancing overall safety during helicopter operations.