Correct answer: 8 : 1
The increase in length is proportional to the applied force and inversely proportional to the cross-sectional area and directly proportional to the length. Since the lengths are in the ratio 1:2 and diameters in the ratio 2:1, the cross-sectional areas are in the ratio 4:1. Thus, the increase in length will be in the ratio (1/4) × 2 = 1:8.