Answer:
B) had known
Explanation:
Plugin the answers into the sentence to see if it makes sense.
They had been known each other for a long time before they finally decided to get married, it doesn't sound right so you can eliminate that answer and choose the other one.
They had known each other for a long time before they finally decided to get married sounds grammatically correct.
Also saying the words out loud helps as well.
Hope I helped :)
