Nunca antes el desafío había sido la conservación de los recursos naturales.
Explanation:
In this sentence we can see a clue of the Spanish tense we should use. The phrase nunca antes which means never before is the clue that you need to complete the sentence with the tense "Pretérito pluscuamperfecto de indicativo" in Spanish. This tense is used in Spanish <em>when you want to express actions that took place before a certain moment in the past. </em>
<em>Había sido</em><em> </em>means<em> had been</em>.
