He tried his best to solve the problem, ______ difficult it was

A.however

B.no matter

C.whatever

D.although

答案及解析： A

解析：

四个选项分别表示然而可是、不论怎样、无论什么和尽管，根据题意选A，他尽了最大努力解决问题，可是困难依旧。