The correct answer is A) If left in the sunlight. Dacron sails, when not in use, can be damaged if left in direct sunlight for extended periods. The ultraviolet radiation in sunlight can break down the molecular structure of the Dacron material, causing it to become brittle and lose its strength over time. This is why it is important to store Dacron sails in a cool, dark, and dry location when not in use, to prevent this type of sun damage. The other options are incorrect because: B) Folding the sails frequently is generally not an issue for Dacron material, C) Washing with soap is usually safe for Dacron, and D) Stowing the sails wet can lead to mildew and other moisture-related damage, but does not directly cause damage to the Dacron fabric itself.