During which condition would pilots most likely encounter severe clear air turbulence?

Severe clear air turbulence is most frequently associated with high altitude jet streams. Jet streams are narrow bands of strong winds in the upper levels of the atmosphere, typically found between 25,000 and 50,000 feet, where the wind speeds can exceed 100 knots. The turbulence occurs as air flows over mountains or along frontal boundaries, causing abrupt changes in wind speed and direction.

Pilots operating near or within jet streams can experience this turbulence, which is often difficult to detect because it occurs in clear air, away from clouds and storms. The turbulence can be quite severe, sometimes resulting in sudden altitude changes and presenting a hazard to aircraft stability and passenger safety.

While other options like clouds at low altitudes, thunderstorms, and stable atmospheric conditions can involve turbulence, they typically do not produce the same degree of severe clear air turbulence found near high altitude jet streams, which is why this option is the correct choice.