Meeting my uncle after all these year was an unforgettable moment ,I will always remember.

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A. that
(答案→)B. one
C. it
D. what
解析：句意：这么多年后遇到我叔叔是我会永远难忘的时刻。这里需要代词指代an unforgettable moment，做它的同位语，one可以泛指前面的可数名词单数，that是特指可数名词单数和不可数名词，it是指同类同物，what不能引导非限制性定语从句，所以选B。