【答案】D
【解析】考查动词。句意：截止到我锁门的时候，我意识到我把钥匙拉家里了。根据时间状语by the time I locked the door，可知 主句用过去完成时。A. had described ”描述“； B. had explained”解释“； C. had forgotten”忘记 “； D. had left”落下“。根据句意，选D。