She still remembers the days _______ she spent in the countryside.
A.while B.which C.when D.during which
我的理解是空格应填spent的宾语，选B，但是先行词是days，那应该用when引导啊，应该考虑哪个

你的第一个考虑是对的，days作为先行词不一定要用when来引导。
就像人作先行词，引导词可以是who也可以是that。
这里要明确的是when是关系副词，而which是关系代词。
从句缺宾语，主语，表语或其他使句子不完整的成分时，要用关系代词来引导。
从句什么都不缺，或者理解为缺状语，这样的情况用关系副词引导。
如:She still remembers the days that/which she spent in the countryside.
She still remembers the days when she spent the whole summer playing in the countryside.