答案是：C
还原为主动语态：
He has made such a good effort in his spare time that his English has improved a lot.这是个结果状语从句．such．．．that结构
如以下例子：
This is such a difficult problem that all of us can't work it out.句中,that是从属连词,引导结果状语从句,在从句中不作成份.
This is such a difficult problem as all of us can't work out.句中,as 是关系代词,引导定语从句,在从句中作宾语.（注意这个句子里面没有上一句的it这个宾语）
窍门：解这类题的关键是看后面有没有差宾语,如差,则用such as 否则用such that．
祝你英语成绩进步!