答案：C

题目分析：考查强调句型和主语从句。强调句型it is/was... that去掉后不影响句意，据此排除B；根据makes可知，that he often fails in exams是主语从句，he often fails in exams是主谓结构，不缺少成分，排除what；that引导主语从句时不能省略，排除D；故选C。