A vessel conducting underwater operations shall display which day shape?

Answer: B — Ball, diamond, ball

Day shapes communicate what a vessel is doing so other ships can take appropriate action. For diving or underwater operations, the specific signal is a vertical line of three shapes: a ball on top, a diamond in the middle, and a ball at the bottom. This Ball-Diamond-Ball pattern tells nearby vessels that divers are in the water and the operating vessel may be restricted in its ability to maneuver, so others should proceed with caution and keep a safe distance. The other options don’t convey diving operations. A single ball indicates something like a vessel at anchor, and a diamond alone or three balls in a line do not communicate the presence of divers in the water.