The correct answer is A: All watertight doors in subdivision bulkheads shall be kept closed during navigation except when necessarily opened for working of the vessel, and in such cases they shall always be ready to be immediately closed. This is the correct answer because it aligns with the key safety principle of maintaining the watertight integrity of the vessel's subdivision bulkheads. Keeping the watertight doors closed during navigation, except when temporarily opened for operational needs, helps to minimize the risk of progressive flooding in the event of damage to the vessel. This practice is a critical safety measure to preserve the ship's stability and trim in emergency situations. The other answer choices are incorrect because they either allow the watertight doors to be kept open during normal operation (B and C), or only require them to be closed when the vessel is anchored (D), which does not address the need for watertight integrity during navigation.