Which type of cloud is created downwind from mountains due to wavelike air action?

The formation of a rotor cloud is specifically associated with the actions of air as it flows over mountainous terrain. When wind encounters a mountain, it is forced upwards, leading to a series of airwaves and eddies in the lee (downwind side) of the mountain. These turbulent air patterns can create instability, resulting in the formation of rotor clouds. Rotor clouds are generally characterized by their turbulent and lumpy appearance, indicative of the turbulent air movements associated with them.

In contrast, stratus clouds typically form in stable air conditions where the atmosphere does not have significant vertical motion, often producing overcast skies. Thunderstorm clouds, such as cumulonimbus, develop in a different context, primarily driven by strong convection in warm, moist air. Cumulus clouds, while they can form in upward air currents, are not linked specifically to winds caused by mountains, but rather to localized convection in unstable air under fair weather conditions.

Thus, the specific dynamics of wave-like air motions generated by the presence of mountains directly lead to the creation of rotor clouds, making this the correct classification in this context.