这句话的that we visited last week 是用来定这家工厂（宾语）的.
而在is this factory that we visited last week中,this factory 是主语,而that we visited last week是修饰宾语的,is this factory that we visited last week中缺少宾语.
所以必须加上宾语,因为主语是this factory ,因此宾语可用代词the one代替.