The best explanation for the changes in cultural exchange that developed during the twentieth century is:
Smartphone technology made global cultural exchange more possible than it had ever been.
This response highlights how advances in technology, particularly in communications, have facilitated unprecedented levels of cultural exchange across the globe. While the other statements touch on cultural exchange, they either misattribute historical context (like the Industrial Revolution starting in the twentieth century) or generalize the phenomenon without providing the specificity that modern technology offers.