Correct Answer & Explanation
Option C
• The phrase 'By the time we reach the harbour' indicates an action that will be completed before a specific point in the future.
• The Future Perfect Tense (will have + V3) is used to express an action that will be completed by a certain time in the future.
• 'Will have left' correctly conveys that the ship's departure will be finished before 'we reach the harbour'.
• Options A (Simple Future), B (Future Continuous), and D (Simple Present) are incorrect because they do not express completion by a future point.