The correct answer is B) One prolonged blast. According to the U.S. Coast Guard regulations, a power-driven vessel leaving a dock or berth is required to sound one prolonged blast on the whistle to indicate its intention to depart. This signal alerts other vessels in the area that the vessel is about to get underway. The other answer choices are incorrect because: A) One short blast is used to indicate a vessel's intention to pass another vessel to the right, C) No signal is necessary is incorrect as a whistle signal is required, and D) Three short blasts is the signal used to indicate the vessel is operating astern propulsion.