We can see that Option 1 is most likely the starting sentence. Now that we know that 1 is the starting sentence we can eliminate choice B and C as they start with 3 and 2 respectively and not with sentence 1.
This narrows down our possibilities to option A and option D.
Now we can see in option A 3 follows sentence 1 but the gap spoken of in sentence 3 has no correlation with political geography of the subcontinent spoken of in sentence 1 , so we can rule out Option A.
Therefore answer has to be option D, as we can also see it elaborates on the change mentioned in sentence 1.