she still remembers the days___she was in the countryside___

C.when; as
...the days when = in which...= in the days
as she was young = when she was young.
通过分解复合句,拆成两个简单句就看的很清楚：
She still remember the days.In the days she was in the countryside as she was young.
看出来了?我们需要一个关系副词指代in the days = in which= when-
d,的前一个选项 during which = in which = when,没有问题,问题是后面不可用that,
b,的第二个选项when 也没有问题,但which 不可以.
只有c,都符合.
她依然记得那些日子,即她年轻时在乡下度过的那些时光/日子.