答案：D

题目分析：句意：如果我说服他不这么做，他就不会犯这么严重的错误了。从主句he wouldn’t have made such a serious mistake.可知这是和过去相反的虚拟语气，条件句应该是：If I had persuaded him not to do so…，这种条件句，可以省略if提前had，所以选D。