Given that BC is the radius of circle B.
Let BC=6 cm.
Take 6cm on the compass and make the origin as B then draw a circle as follows.
Mark a point on the circle as C and join B and C.
Take an angle 90 at B and draw a ray from B. name as BX.
Take an angle 90 at C and draw a ray from C. name as CY.
Draw a line above the circle parallel to BC on BX and CY.
Name intersection points as A and D respectively.
Hence this is the required rectangle ABCD with BC is the radius of circle B.