本题选 A 或B都可以。即回答 had better 既可用 had better,也可以只用 had。
但是,如果是作否定回答,则应用 had better not,而不用had not(因为除用于否定疑问句或反意疑问句等少数情况外,had better 的否定式通常应在 had better not 后面加上not)。如下面一句摘自《薄冰高级英语语法》:
—We had better go now. 我们最好现在走吧。
—Yes, we had. 是,我们最好走吧。(或we’d better 或we better had)
—No, we’d better not. 不,我们最好别走。(not 须放在 had better 之后)