the famer did what could __that tree.A,saveB,to save C,savin

当然可以》不过提醒你,这句话应该是这样的,不过当然填B.
The farmer did what he could to save that tree.
为什么用 to save?what he could 作为 “他尽力” 如果要再接一个动词,就一定用不定式.
例子》 I did what I could to finish the art homework.
what I could 表我尽力.
在英语不定式经常用在第一个动词后面.
I ordered him to go to the seashore.

The famer did what he could to save the tree.所以选B
整个句子是个宾语从句。farmer 是主语，did是动词，what he could 是宾语，to save the tree 是表目的的补语

B
“Famer” is the subject,then “did” is the verb, "what he(少了个he吧) could" is the object. and the preposition "to" or phrase “in order to” is used to show purpose.thus，it should be "to save"

B