He could have come to help us ,but_____.a.he didn't b.he hasn't c.he hadn't d.he can't为什么选a不选b如果后面写反义疑问句是不是用didn't he?

could have come to是虚拟语气，用过去时he didn't 表过去没有做的事实。

虚拟语气，后句时态过去

这是一个虚拟语气中的常考句型,要记住.could have done的意思是“本可以做某事（而实际上没做）”.句型：Sb could have done sth,but + 一般过去时.的意思为：某人本可以做某事的,但（实际情况是）…….由于后句讲述的是真实情况,故用一般过去时.
关于你的补充问题：
He could have come to help us后的反意疑问句的tag理论上是didn't he,只是此句中He could have come to help us应该是一个确定的判断,一般不会用反意疑问句形式.
如果是猜测,则一定是didn't he.请看下列例句：
He could not have come to help you,did he?
He must have come to help you,didn't he?
He might have come to help you,didn't he?
祝周末愉快!