He tried his best to slove the problem,-A---difficult it was

梦之南草的回答简直可笑,只知道死记硬背书本知识.
however此处是“无论多么”的意思,接形容词
whatever 无论什么,本身就是名词,不能接其他成分
no matter 后面要加how才构成“无论多么”的意思,和however用法相同
although后的语序不能倒装,应是although it was difficult