--It's very important for us to keep a good relationship ___

选C.
keep relationship between 是一个常用词组,表示与……保持某种关系.
“relationship”n.关系,关联;人际关系[U][C][(+between/to/with)]
例如：
The children learned about the relationship between the moon and the tides.
孩子们弄懂了月球与潮汐的关系.
另外,among是表示处于两个以上的事物当中这样一种状态.
in the middle of是指位置或时间在两个之间.