Although it was really a difficult task he decided to try his best to success.A. challen

问题补充：

Although it was really a difficult task, he decided to try his best to success.

A. challenge

B. host

C. approve

D. achieve

答案：

A. challenge

B. host

C. approve

(答案→)D. achieve

解析：句意，尽管这的确是一项很艰巨的任务，但是他决定尽他最大的努力取得成功。challenge意思是挑战；host意思是主办；approve意思是赞同；achieve意思是取得。根据句意分析，本句要表达的意思是取得成功，故选D。