D.
Topography, like in India, the Deccan Plateau served as a barrier separating Indian culture into different sectors.
Distance, I think it's plain to say that distance clearly prevents cultures from mixing or creating a new culture, because they arent close enough to do so.
And finally, man-made laws undoubtedly prevented cultures and subcultures from intermingling, for example in Europe, during the Middle Ages, the Muslims tried to conquer parts of Europe like Spain, and when Charles "the Hammer" Martel knocked them back down to northern Africa, the southern tip of Spain, and small river valleys by Gilbatrar became very Islamic, but if it was prevented with harsh laws, Spain wouldve never had a Muslim influence at all.
