The correct answer is D) danger to crew if it parts. The main disadvantage of using three-strand nylon line for towing is the danger it poses to the crew if the line parts under tension. Nylon line has excellent shock-absorbing abilities, but if it breaks, the sudden release of energy can cause the line to snap back violently, potentially injuring or even killing crew members. This danger makes it a less desirable choice for towing compared to other line materials that are less prone to sudden failure. The other answer choices are incorrect because: A) three-strand nylon line is not particularly susceptible to rot, B) the shock-absorbing properties are actually an advantage, and C) the inherent weakness is not a primary disadvantage compared to the risk of crew injury if the line parts.