Q: Does "which" refer to the Chinese buzzwords or the social changes and cultures? 
The Chinese buzzwords usually reflect the social changes and cultures, some of which are increasingly popular with the foreign media.

I saw this sentence in an exam. What is the antecedent of " which", the Chinese buzzwords or the social changes and cultures?

A: The (sloppy) writer is almost certainly actually referring to the Chinese buzzwords becoming more popular with foreign media covering the country's culture. They also mistakenly describe "social changes and cultures" when they really mean "social and cultural changes".
Misplaced modifiers are extremely common in English writing and even moreso in informal writing about popular culture. The "cultures" thing is more bizarre and, in my own experience, similar to mistakes that pop up when Chinese English teachers try to "correct" "mistakes" in foreign sources like articles from the Voice of America and the BBC.
Answering this correctly on your test would depend on knowing exactly how bad your teacher's English actually is and how focused they've been recently on getting you to notice modifier placement. They may want you to follow the grammatical rule ("which" should modify the nearby "social changes and cultures") instead of noticing that it makes no sense for "changes and cultures" to become more popular with foreign media.
