答案：C

题目分析：句意：就是他考试经常不及格让他的父母为他担心。首先第二个填that为强调句，It is that中的that 为固定搭配，去掉It is that来看，这是句子为That he often fails in exams makes his parents worried about him在这一句中That he often fails in exams 为主句，这是个强调的主语从句，选C。