The correct answer is B) a line diagram of the vessel transfer piping. The transfer procedures required to be followed on tankships must contain a line diagram of the vessel transfer piping. This is because the line diagram provides a visual representation of the piping system used for cargo transfers, which is critical for ensuring the safe and efficient execution of transfer operations. The other answer choices are incorrect because they do not directly relate to the required contents of the transfer procedures. Listing the duties of personnel (A), providing a crew list (C), or listing ports of discharge (D) may be important for other aspects of the operation, but they are not the primary requirement for the transfer procedures.