Choose the right form of the verb:
Exhausted from their journey, the travelers had rested briefly before they _______ their adventure.

• কোন sentence এ 'before' থাকলে, 
Before এর প্রথম অংশ Past Perfect Tense এর হলে পরের অংশ Past Indefinite Tense হয়। 
যেমন- They had eaten dinner before the movie began.
- She had visited that museum before it closed for renovations. 

• কোন sentence এ 'after' থাকলে, 
After এর প্রথম অংশ Past Indefinite Tense এর হলে পরের অংশ Past Perfect Tense হয়। 
যেমন - He arrived at the airport after the flight had departed.

So, Complete Sentence - Exhausted from their journey, the travelers had rested briefly before they continued their adventure.

Source: Advanced Learner's English Grammar & Composition by Chowdhury & Hossain.