What type of wind blows in the direction of travel and is unfavorable during takeoffs and landings?

The correct answer is that a tailwind blows in the direction of travel and is unfavorable during takeoffs and landings. When an aircraft is taking off or landing, it requires lift to become airborne or to safely reach the ground. A tailwind, which is wind blowing in the same direction that the aircraft is flying, reduces the effectiveness of the aircraft's lift by increasing the ground speed but not the airspeed needed for lift. This can result in longer takeoff distances and can compromise safety during landings, as it can lead to reduced controls and increased landing speeds.

In contrast, headwinds would provide an advantage during takeoffs and landings as they increase the relative airspeed over the wings, enhancing lift. Crosswinds can pose challenges in directional control but don’t affect the airflow over the wings in the same detrimental way a tailwind does. Finally, thermals relate to rising air that can aid in gliding but are not directly applicable to the concepts of takeoffs and landings concerning wind direction.