What type of clouds form at altitudes between 6,500 and 25,000 feet?

Altostratus clouds are characterized by their formation at mid-level altitudes, typically found between 6,500 and 25,000 feet. These clouds are often gray or blue-gray and can cover the sky, frequently indicating that rain or snow may occur. They are formed in stable atmospheric conditions where moist air rises and cools, leading to the condensation of water vapor.

Stratus clouds generally form at lower altitudes and are more uniform and flat. Cumulus clouds are typically associated with vertical development and can grow to great heights but usually appear as puffy and white clouds in the lower atmosphere. Stratocumulus clouds also form at lower altitudes, appearing as lumpy and layered formations.

Thus, the defining characteristic of altostratus clouds is their mid-level altitude, making them the correct choice in this context.