I believe the right correction for the sentence would be "The decrease in members from the clubs are affecting". The first option wouldn't be correct because the word "decreasing" doesn't fit right grammar wise in the sentence. Option two wouldn't make any sense put together with the sentence, it would just sound completely out of place. Option three has the word "were" as in it was affecting the club, not presently affecting the club. And obviously, this sentence needs improvement so option five is out of the question.
