Which of the following is a sign of very strong turbulence?

The presence of standing lenticular altocumulus clouds is indicative of very strong turbulence. These clouds form when moist air flows over mountain ranges or other geographical features, leading to wave-like patterns in the atmosphere. As the air rises and descends in these waves, it can create significant turbulence, particularly in the vicinity of the clouds.

Standing lenticular clouds are distinct due to their lens-like shape and can remain stationary as the turbulent flow is present in the surrounding air. This characteristic demonstrates that there is a lot of energy and movement in the atmosphere, which is a key sign of strong turbulence.

In contrast, cumulus clouds generally indicate moderate convective activity, while cirrus clouds are high-altitude clouds that typically suggest fair weather rather than turbulence. Stratus clouds are associated with stable air and generally smooth flying conditions. Thus, the presence of standing lenticular altocumulus clouds is a clear indicator of the turbulence's intensity and potential hazards during flight.