Concept:I'tikaf is a spiritual retreat observed in the mosque, especially during the final part of Ramadan.Explanation:The Prophet (SAW) regularly practised I'tikaf during the last ten days of Ramadan.These days are special because Laylatul-Qadr is likely to occur within them.The first ten days of Ramadan and the days of Dhul-Hijjah are not the specified time for I'tikaf.Therefore, the appropriate time is the last ten days of Ramadan.Answer:Last ten days of Ramadan.Correct option: C.