Explanation:

It is our responsibility to leave a green and clean world for our descendants.
option 1 and option 3, both spellings are correct, but we choose option 3 as to make a sense.
This is a traditional difference which is at times lost out in modern usage. Descendant, used as a noun, refers to someone who descends from someone else. Descendent, on the other hand, refers to the act of moving downward. Descendent is a word that is used in place of descendant now, though its usage is less common.