The correct answer is A) stretches. Nylon line can be dangerous because it stretches significantly under load. This stretching behavior means the line can suddenly recoil with great force when it reaches the end of its stretch, which can lead to injury if the line snaps back towards the user. The stretching property of nylon makes it less suitable for certain applications on board a vessel compared to other line materials that are less elastic. The other answer choices are incorrect because: B) kinking when wet is not the primary safety concern with nylon line, C) nylon line does not typically break down when wet, and D) the elasticity of nylon is actually a key reason it can be dangerous, not that it lacks elasticity.