Is this factory(the one ) you visited last week?
Is this factory(where) you visited last week?
Is this the factory(that/which) you visited last week?前面有this 了，为什么还要用the呢？Is this factory(the one ) you visited last week?改为陈述句： This factory is the one (that) you visited last week.。。前面有this了，后面还要有the 干嘛。。