The correct answer is D) discharge liquid left in the cargo tanks after the main pumps have discharged the bulk. The main function of a stripping system on a vessel is to ensure complete discharge of the cargo from the tanks. After the main cargo pumps have removed the majority of the cargo, the stripping system is used to extract the residual liquid that remains in the bottom of the tanks. This helps to maximize the amount of cargo that can be offloaded, minimizing the amount of cargo that must be disposed of or dealt with as tank residue. The other answer choices are incorrect because they do not accurately describe the primary purpose of a stripping system. Increasing the loading rate, maintaining cargo temperature, and disposing of vapors are not the main functions of a stripping system.