The correct answer is A) gastight bulkheads. In a tank vessel carrying flammable liquid cargo, the cargo pumps must be isolated from sources of vapor ignition, such as machinery spaces or other ignition sources. Regulations require that this isolation be achieved through the use of gastight bulkheads, which are walls constructed to prevent the passage of gas or vapor. This ensures that any potential vapor leaks from the cargo system cannot enter other areas of the vessel where ignition sources may be present. The other options are incorrect because passageways or living quarters (B) would not provide the necessary isolation, cofferdams (C) are not specifically required for this purpose, and general cargo spaces (D) are not the appropriate means of isolating the cargo pumps.