What effect does a tailwind have on an aircraft?

A tailwind has the effect of increasing the ground speed of an aircraft as it moves in the direction of travel. When an aircraft is flying with a tailwind, the wind is pushing the aircraft from behind, effectively adding to the speed at which the aircraft is moving relative to the ground. This means that although the indicated airspeed might remain constant, the ground speed increases, which can reduce the time taken to reach a destination and can lead to fuel savings as well.

The other options do not accurately reflect the effects of a tailwind. A tailwind specifically contributes to increased speed, while the decreasing speed and turbulence would be associated with headwinds or other flight conditions. Altering altitude is not a direct effect of wind; rather, altitude changes are typically managed by pilot input and atmospheric conditions, not simply the presence of a tailwind.