He looks sad because there are so many problems -----------.

俊狼猎英团队为您解答.
C. remaining to be settled 为正确选项.
根据句子的意思,problems 后面的成分是要做定语的,所以这里要用remain的现在分词remaining做后置定语,因为做定语时,remain是不能用过去分词的,它相当于一个系动词,没有被动语态；problems是被settle的,故用被动语态settled.
祝你进步