The radius of the Outer circle (with r₂) is 6 and the radius of the inner circle (with r₁) is 2. When objects are similar, they have a proportional relationship. If we take the outer circle and squeeze by one third, we get a radius of 6 x 1/3 = 2. On the other hand, if we start with the inner circle and stretch it by 3, we get a radius of 2 x 3 = 6.
Thus, to make r₂ similar, we use a scale factor of 1/3. The third choice is best.
