This is the sentence:
I went to see it because I was/had been told it was a good film by all my friends.
was, but the correct answer is
had been. Can someone explain why is it so?
I went to see it because I had been told it was a good film by all my friends.
You have two separate events here:
The first event:
The second event (which is, in fact, the next event):
Your sentence indicates that the second event happened before the other one as after your friends told you about the film, you went to see it (as a result).
Therefore, and since the two events occurred at some time in the past, what happened first in the past takes the past perfect (in the passive voice because of your sentence structure), and the event that happened later in the past takes the past simple.