The correct answer is C) Void or empty space separating two tanks. A cofferdam is a void or empty space between two tanks or compartments, usually used to provide a physical separation and prevent the contents of one tank from mixing with the contents of the adjacent tank. This separation is important for safety and environmental reasons, as it helps prevent leaks or spills from one tank from affecting the other. The other answer choices are incorrect because they do not accurately describe what a cofferdam is. A cofferdam is not an area for loading cargo (A), an opening in the deck (B), or a tube fitted to an ullage hole (D). These are different components or features of a vessel, but they are not the definition of a cofferdam.