The correct answer is A) cotton. White line, also known as cotton line, is a type of rope or line made from cotton fibers. Cotton is a natural fiber that has been commonly used in the production of various types of rope and line used in maritime applications, including on US Coast Guard vessels. The cotton fibers are spun and woven together to create a strong, durable line that is well-suited for use on ships and boats. The other options, B) manila, C) sisal, and D) hemp, refer to different types of natural fibers that may be used to make ropes and lines, but they are not the specific material used to make "white line" as referenced in the question.