B
He used the room as his first laboratory.
that/which 可代替the room,但是这句话不用加介词in
the one用于表特指,但是room前面的the已经可以表示特指了,所以不是the one,如果room前面没有the,则用the one,即Is this room the one he used as his first laboratory?
who只能指代人,不说了