At middle latitudes, the base of an altostratus or altocumulus cloud would generally be found between:

The correct answer is: B. 6,500 and 23,000 feet

Altostratus and altocumulus clouds are mid-level clouds, which means they are found at an altitude of 6,500 to 23,000 feet (2,000 to 7,000 meters). They are typically white or gray in color and can be smooth or lumpy. Altostratus clouds often cover the entire sky and can produce a light drizzle or snow. Altocumulus clouds are often associated with fair weather.

Option A is incorrect because altostratus and altocumulus clouds are not found at altitudes below 6,500 feet. Option C is incorrect because altostratus and altocumulus clouds are not found at altitudes above 23,000 feet. Option D is incorrect because altostratus and altocumulus clouds are not found at altitudes above 43,000 feet.