Concept:Hajj (pilgrimage to Makkah) is an obligatory act in Islam performed once in a lifetime by those who are able.Explanation:The Quran and Islamic teachings state that Hajj is required only once in a Muslim’s lifetime, provided the person has the physical and financial ability.Performing it more than once is voluntary and extra, but the obligation is fulfilled by doing it one time.Answer:C. once in a life time