How did the ancestral puebloans use their surrounding environment to create effective housing solutions?

Ancestral Puebloans built stone houses against canyon halls, is the right answer.  

Explanation:

The Native American cultures of various regions of the United States such as Southeastern Utah, northwestern New Mexico, northeastern Arizona and southwestern Colorado, are known as the Ancestral Puebloans. These people made good use of the resources available in their surroundings. This fact may be proven by their housing structures. They made use of stone, adobe mud and other available materials to built apartments- like structures. These structures were usually created on the canyon walls’ sides.

letter a is the answer