The correct answer is C) Polypropylene. Polypropylene is a type of synthetic fiber that is known for its excellent buoyancy and ability to float on water. This makes it a preferred material for marine lines and ropes used in various Coast Guard operations, as it can keep the line afloat and prevent it from sinking. The other options, such as nylon, old manila, and Dacron, are not as buoyant as polypropylene and are more likely to sink in water, making them less suitable for applications where the line needs to float.