Answer:
The correct answer is:
(B) them
Step-by-step explanation:
(A) would makes sense if the sentence was talking about only one concert.
(C) wouldn't make sense because you use the word "their" when you talk about someone else owning an object.
(D) is talking about people not concerts so that wouldn't make sense either.
Therefore, the answer is (B) them.
Hope this helps!