It was the training that he had as a young man ----- led to

the training that he had 中的 that he had 是作为定语从句存在的.
as a young man 是状语,表示状态.
整个句式是一个强调句,总句的意思是是他在年轻的时候收到的训练让他获得了成功,强调训练,强调句式用that,所以第二个空也是that
答案为A
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