So, the area of a circle is a=pi r^2 so do that and get a=pi(4) and since the circle touches the edge of the rectangle, the rectangle is 4 cm tall, so 4*8 is 32 and pi(4) is about 12.56, so 32-12.56 is 19.44, and I believe that is the answer. (19.4)
While a "given" is taken as true without proof, and a "problem" sometimes involves an assertion that can be proven, the answer choices involving these terms are not correct here.
The dictionary definitions of "postulate" and "theorem" apply. It is useful to learn the meaning of these terms so you can understand problems and descriptions that use them.