The correct answer is B) one short blast. According to the Inland Navigation Rules, when two power-driven vessels are crossing within half a mile of each other, the vessel that has the other on its own starboard side (in this case, Vessel "B") must sound one short blast on the whistle to indicate its intention to hold course and speed. This allows the other vessel (Vessel "A") to take the appropriate action to avoid a collision. The other answer choices are incorrect because: A) two short blasts is the signal used to indicate that the vessel is altering course to starboard, C) three short blasts is the signal used to indicate that the vessel is operating astern propulsion, and D) two prolonged blasts followed by two short blasts is the signal used to indicate that the vessel is maneuvering to get underway.