选D.
把句子改为陈述先：
This museum is _____ you visited a few days ago.
显然you visited a few days ago是要做为宾语定语从句来使用的,但是This museum is 缺一个宾语,因此要在这里填上宾语,那么只有the one是适合的.这里其实还省略了一个that,即This museum is the one (that) you visited a few days ago.当宾语不仅是主句的宾语,还是定语从句的宾语时,that可以省略.
而且还隐藏了一个知识点,即one是加了定冠词的,这时不能用which,只能用that.