It really depends on the person and what they think is more important -
A: You could show that the 2008 Olympics was Michael Phelps' "last hurrah" as it were, as he did not do as well in the 2012 games, BUT the paper is on the 2008 games and not Phelps himself
B: The proof of the use of steroids could be used to make a point that perhaps the security needs to be stepped up, or that in the modern world, sport is becoming too unfair as new drugs are invented because people feel the need to cheat for the cash prizes.
C: This is not really an essay topic, but perhaps you could combine it with B as a controversial point.
D:You could use this point as proof that the bar is being raised every year, and that talent is being found everywhere, better talent, even from poorer countries that you wouldn't expect. You could also tie in Jesse Owens.
Personally, I would use B as drugs is a hot topic around the world, but it is ultimately up to Jacob.
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.