B) ensure
The correct option is B) ensure because the sentence requires a base form verb (infinitive without 'to') after the modal auxiliary verb structure "let's … and _________ a greener tomorrow."
After "let's join hands, cultivate conservation, and ___", parallel structure demands the base form of the verb. "Let's" = "let us", which is followed by bare infinitives. All three verbs — join, cultivate, ensure — must be in the same base form. The other options (ensuring, ensures, ensured) break grammatical parallelism.