C
意思是 直到很晚他才意识到已经很晚了,改回家了
也就是说很晚的时候他才意识到/或者说 “他没有意识到”这个动作一直发生,直到“很晚”了才结束
这是强调句,还原之后应该是 He hadn't realized it was too late to return home until it was dark
强调的时候把not 和until提前
也可以说成It was until dark that he hadn't(didn't) realized it was too late to return home但很少有人这么说