The correct answer is C) the channel boundaries. Under a single-span fixed bridge, red lights displayed indicate the edges or boundaries of the navigable channel that vessels must follow. This is a standard regulation for the U.S. Coast Guard to mark the safe passage route for vessels passing under the bridge. The other answer choices are incorrect because: A) red lights do not indicate approaching traffic from the other side, B) red lights do not mean vessels must stop, and D) red lights do not indicate the bridge is about to open, as that would typically be signaled by different bridge opening lights or signals.