your advice I would have had no trouble with my papers.A. Had I followedB. If I followed

问题补充：

your advice, I would have had no trouble with my papers.

A. Had I followed

B. If I followed

C. If I have followed

D. Once I followed

答案：

(答案→)A. Had I followed

B. If I followed

C. If I have followed

D. Once I followed

解析：根据would have had可知, 这是是一个表示与过去事实相反的假设。if条件从句用过去完成时had done，又知if条件句中如有were, should, had，可以省去if，并使用倒装语序, 即were, should, had+主语+谓语动词。故答案为A。