He still remembers the days ______ he spent with your family A. when B. where C. that D. on which

spend在这里是及物动词。。the days 是做后面spent的宾语的，所以所填项是which或者that。。。 句子还原过来就是He still remembers the days. 什么样的days呢？ he spent（the days）with your family .

A 主句和从句都要保证句法完整。 如果是A，则应该变成He still remembers the days when he spent（summer vacation） with your family.（）里必不可少。。当然没人会这么表达，有重复之嫌疑。
D 你认为on which在这里是介词提前， 那我们把它还原到后面。He still remembers the days which he spent on with your family? 你自己就看出不对了吧。
希望能帮到你

这里因为the days 是做后面spent的宾语的，所以只能是which或者that，而如果是完整的句子的话，就因该是on which或者when了
I remember the days on which/ when we had a good time last sunday

A 如果是when，应为：he still remembers the days when he spent with your family yeas ago.
D spent with sb.on days 明显不是短语，spend on 为花费的意思，度过时间为spend some times