The correct answer is B) the transfer of heat across an unobstructed space. Radiation is the transfer of heat through electromagnetic waves, which can cause a fire to spread by transferring heat across an unobstructed space. This is in contrast to the other options, which involve the physical movement of heated liquids, gases, or the conduction of heat through a solid material like the ship's metal. The other options are incorrect because they do not accurately describe how radiation specifically can cause a fire to spread. Burning liquids, heated gases, and heat conduction through metal are other mechanisms of fire spread, but they are not the primary way that radiation acts.