Jan and Daniel <u><em>had been</em></u> on an adventure together before the polar challenge.
Explanation:
The use of "had been" or "were" depends on the time frame of the event. "Had been"(past perfect continuous) is used for a past event that is also applicable for the present or current event. Whereas, "were" (past tense of ‘be’) is used for a single past event that no longer applies to another event.
In the given sentence, Jan Meek and her son Daniel are together on the polar challenge. And they had been on an adventure before the polar challenge, which justifies the use of "had been". This shows that the duo's adventure is not a one-time event but that they were together for a previous adventure and are had gone on another after that previous one.
