The correct answer is B) Combines individual vent lines into a main. A vent header is used to combine the individual vent lines from multiple fuel tanks or other compartments into a single main vent line. This allows for more efficient venting of the system and helps to prevent the buildup of pressure or vacuum within the fuel system. The vent header consolidates the venting to a single point, rather than having multiple individual vent openings. The other answer choices are incorrect because they do not accurately describe the primary function of a vent header. A vent header is not specifically designed to prevent flame passage, catch overflow, or prevent water entry - those are typically handled by other components in the fuel system.