The correct answer is B) in the pump room. The pump room on a tanker is the most likely location for a cargo fire to occur due to the presence of highly flammable cargo, potential ignition sources, and the confined nature of the space. The pump room houses the cargo pumps and associated piping, which increases the risk of leaks, spills, and vapor accumulation - all of which can lead to a fire. The other options are less likely because the main deck manifold (A) and vent header (D) are more open spaces, while the midships house (C) is typically a living/accommodation area rather than a cargo-handling space.