The volume of the smaller room is B. 768 cubic feet.
Step-by-step explanation:
Step 1:
The entire length for both rooms is 20 feet. As the large room has a length of 12 feet, the length of the smaller room is
18 feet long.
The width of the prism is 12 feet and a height of 8 feet.
The volume of a rectangular prism is determined by multiplying its length, its width, and its height.
The small room's length = 8 feet,
The small room's width = 12 feet, and
The small room's height = 8 feet.
Step 2:
The volume of a rectangular prism 
The volume of the smaller room 
So the volume of the smaller room is 768 cubic feet which is option B.