To maintain a nice relationship both sides are advised to focus on they have in common.

问题补充：

To maintain a nice relationship, both sides are advised to focus on they have in common.

A. what

B. that

C. which

D. where

答案：

A. what

B. that

C. which

D. where

解析：句意：为了维持一个良好的关系，双方被建议专注于共同的东西。介词on后面接的是宾语从句，宾语从句中缺少宾语，用what引导宾语从句，所以选A。