The question concerns the correct use of although as a subordinate clause. In the options, subordinate clause deals with the truth or fact, i.e, theory here is a fact. Therefore, we have to use present simple in the subordinate clause. According to this result, we cannot use past simple in the main clause. So that, we are left with two options - B and D. As we mentioned, it is a fact that the theory exists, we cannot claim that it is continuing. Therefore, the main clause cannot be in present continuous. The correct option is B.
