This museum is the one that some German friends visited last Wednesday.
visit 是及物动词 所以要有名词或者代词做宾语
A This museum is that some German friends visited last Wednesday.显然不对 因为缺先行词和宾语
B This museum is where some German friends visited last Wednesday.不行是因为 visit 后面不能加副词 where
C This museum is in which some German friends visited last Wednesday.还是不对因为这里有介词in visit是及物动词 不需要加介词的 还有 没有先行词就有which 不妥