I still remember the time -we spent together at xishan mount

考察的是定语从句,BC可以直接排除,纠结在A和D。这句话的先行词是the time,因为后面跟的是spent这一动词,所以你不能把the time 看成是时间,而应该看成是某物。spend sth on sth或者spend sth(in) doing sth。所以用which代替先行词。换一种情况,I still remember the time -we “are” together at xishan mountain last year。这时候就要用when。解释得不够专业,不知道你能不能明白~