Correct Answer & Explanation
Option C
• This sentence describes an action that will have been ongoing for a duration ('for two hours') up to a specific future point ('when they reach there').
• The Future Perfect Continuous Tense (will have been + V1-ing) is used for actions that continue up to a future point.
• 'Still will have been sleeping' correctly indicates an action that will have been in progress for a specified duration by the time another future event occurs.
• Options A (Simple Present), B (Future Continuous), and D (Simple Future) are incorrect as they do not convey the idea of an action continuing for a period up to a future point.