is this research centre _____the foreign guests visited last week.

把句子还原成陈述句就是：This research centre is the one (that ) the foreign guests visited last week.
the one做定语从句的先行词,后面的that可以省略,因为先行词在从句中作宾语,可要可不要.如果选A或B的话,句子没有先行词.C的话,the foreign guests visited后就没有宾语了,where等关系副词引导定语从句时后面的从句是完整的句子.
还有一个相区别的句子：Is this the research centre that/which the foreign guests visited last week 同理,我们可以吧它还原成陈述句就是：This is the research centre that/which/省略 the foreign guests visited last week.此句中有先行词the reasearch centre,所以用关系代词“that/which/不填”都可以的.