Which index lists onboard ships drawings and blueprints?

The Ship Drawing Index (SDI) is the correct choice because it serves as the primary reference for all onboard drawings and blueprints related to a ship. The SDI organizes and categorizes these essential documents, allowing for easy access and retrieval when needed. This is crucial for maintenance, repairs, and modifications, as the drawings provide crucial information regarding the ship's systems, layout, and specifications.

Other options may reference related concepts, but they do not specifically serve the purpose of indexing drawings and blueprints in the same comprehensive and organized manner as the Ship Drawing Index. Understanding the role of the SDI helps ensure that personnel can efficiently locate and utilize technical drawings, which is vital in the field of naval operations and maintenance.