答案是A.
1 What was it that made him successful in his caneer?------- 这是强调句的特殊疑问句形式.
2 Do you know + 强调句的特殊疑问句形式(what was it 要变成陈述句语序),这样就成了你的句子：
Do you know what it was that made him successful in his caneer?