The correct answer is C) a yellow diamond. The draw span of a floating drawbridge is typically marked with a yellow diamond to indicate the presence of the draw span. This is in accordance with the United States Coast Guard regulations and the Inland Navigation Rules, which govern the requirements for marking structures that obstruct navigation. The other answer choices are incorrect because: A) two white lights are used to mark the center of the navigable channel, not the draw span; B) three red lights on each side of the draw are not the standard marking for a floating drawbridge; and D) flashing blue lights are used to mark certain types of bridges, but not specifically for floating drawbridges.