The correct answer is A) When stowed wet. Canvas sails can be damaged if they are stowed while still wet or damp. The moisture can cause mildew and rotting of the fibers, leading to weakening and deterioration of the sail material over time. Leaving canvas sails in the sunlight (B) can also cause damage through UV exposure, but this is not the primary cause of damage when the sails are not in use. Frequent folding (C) and washing with soap and water (D) are not inherently damaging to canvas sails when they are properly dried and maintained. The main culprit for damage to unused canvas sails is allowing them to be stowed in a wet or damp condition.