Which of the following is a cause of turbulence?

Turbulence is primarily caused by factors that involve changes in wind speed and direction, as well as variations in atmospheric conditions. Wind shear refers to the sudden change in wind speed or direction over a short distance, which can create unstable air pockets leading to bumpy flight conditions. Temperature inversions occur when a layer of warmer air traps cooler air beneath it; this can affect vertical movement in the atmosphere and result in turbulence.

While clear sunny skies and high humidity do not typically lead to the kind of turbulence referred to in aviation safety contexts, they can influence flying conditions under specific circumstances. Cloud formations can also cause turbulence but are not the sole factor responsible. Therefore, wind shear and temperature inversions are the most significant contributors to turbulence, making this choice the correct answer.