Final answer:
The adverb that best completes the sentence is 'prudentemente'. It conveys the idea of using resources in a responsible and careful manner to save the planet.
Step-by-step explanation:
The adverb that best completes the sentence is prudentemente. This adverb means 'prudently' or 'cautiously' in English. It is the most appropriate choice in the context of wanting to save the planet as it conveys the idea of using resources in a responsible and careful manner to achieve the goal.
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