Final answer:
One prolonged blast of 4-6 seconds on a ship's signal device typically indicates that the vessel is underway and making way through the water, and does not have a specific meaning related to direction or other vessel positions.
Step-by-step explanation:
One prolonged blast that lasts 4-6 seconds on a ship's whistle or horn signifies that the vessel is underway and making way through the water, according to international maritime signal codes. This sound signal is used to inform other vessels of the ship's presence and intentions and is not specifically used to signal a turn, backing up, or the presence of another vessel on the starboard side. The meaning of one prolonged blast does not point directly to any of the specific actions listed in the options (a to c), and so the correct answer would be (d), it has no specific meaning in indicating direction or movement such as turning right, indicating backing up, or warning of a vessel on the starboard side. It is rather more generic, indicating the vessel's status of movement.