It is high time that he ( )immediately.A.leave B.should leav

B和C都正确
It is high/about time that 正是干……的时候了high/about 只是对time 强调
从句谓语动词要用过去式或用should加动词原形,但should不可省略.
例如
It is time that the children went to bed.
It is high time that the children should go to bed.
所以本题正确答案有B和C
It is high time that he left / should leave immediately.

选B
。。。。固定用法