翻译：客人们离开之后,她花了她能够花的（尽可能多的）时间清理房间.
解释：
1. as she could = as she could spend,该句为定语从句,其中as代替先行词time,并在定语从句she could spend中充当spend的宾语.
因为先行词time被as所修饰,所以其后的定语从句的关系代词必须要as,不能用that/which.
所以,句子主干还是she spent much time ___ the rooms.
2. 剩下的就是一个固定结构的用法了：spend some time (in) doing sth,其中的介词in经常省略.
所以正确答案只能是D.