SAT Mathematics Practice Test 4

Since segment AB is a diameter of the circle, it follows that arc ADB is a semicircle. Thus, the circumference of the circle is twice the length of arc ADB which is 2(8π) = 16π. Since the circumference of a circle is 2π times the radius of the circle, the radius of this circle is 16π divided by 2π, which is equal to 8.
Choices A, B, and D are incorrect and may be the result of losing track of factors of 2 or of solving for the diameter of the circle instead of the radius. For example, choice D is the diameter of the circle.