A = Quickly
B = Economically
C = Easily
D = A lot / Often
Si nous voulons sauver la planète, il faut que nous utilisions les ressources naturelles = If we want to save the planet, we must use natural resources
I think the answer is B. Economically.
Si nous voulons sauver la planète, il faut que nous utilisions les ressources naturelles beaucoup.
(I apologize if this is wrong. I graduated my French unit awhile ago.)