回答语完整的是：Yes ,I could have forgotten to lock the door.因为这句子里的谓语有两个助动词could have ,所以不能省略成Yes ,i could.这样会产生歧义,因为这样可以理解这句是I could forget to lock the door,与原句不符.
所以这种有两个助动词的谓语的省略应该省略到第二个助动词.
C和D两个答案与前一句不搭配,不一致.