The correct answer is C) by day, three black balls in a vertical line. The International Regulations for Preventing Collisions at Sea (COLREGS) specify that a vessel aground must display, by day, three black balls in a vertical line as a visual signal to indicate its status. This requirement applies to both international and inland waters. The other answer choices are incorrect - option A describes the anchor lights and lights required for a vessel at anchor, not a vessel aground; option B describes the sound signal required, not the visual signal; and option D is incorrect as it combines multiple different signals, only one of which is the correct visual signal for a vessel aground.