I believe the answer is the second one. As you can clearly see, there is no drastic contrast between the side going vertical and the side going horizontal, so they're approximately the same length. Then the diagonal one is notably longer than the other two. You really can't choose the others because not all of them have a different length than another, they're obviously not of the same length, and the diagonal line isn't quite long enough to be the sum of the other two.
The constant of proportionality is the ratio between numbers that expresses the rate at which the numbers increase or decrease. These two numbers are directly proportional when they increase and decrease at the same rate.