Phew! Typical LSAT wordiness - you will likely never see such a confusingly worded passage on the GMAT. But anyway, it's a good way to build your reasoning and logic to be able to weed out trap answers and see through the BS.
Ignore all this "Medieval" and "manuscript" nonsense and see what this statement is logically saying:
Group A translates Greek books into Arabic when needed.
Group A has poets and philosophers.
Philosophers were very interested in Book B.
Conclusion: Poets were not interested in Book B.
Evidence: If they were interested in Book B, they would have also been interested in Book C - but book C has only been translated recently.
STRENGTHENER:
A. Group A had Book C. Hmm. Seems so stupid doesn't it ? But heck, it's the only one that tells us that Book C was actually not translated then EVEN THOUGH they had it. This helps us rule out one possibility, that the poets didn't even have Book C so how could they have shown interest - so by ruling out something that negates the argument, this choice supports our argument. Not a common thing on the GMAT but they love these on the LSAT evidently
B. There are already books in Arabic that are similar to Book C. Maybe this is why C wasn't translated? This seems to weaken our conclusion rather than strengthen it because it attacks the evidence!
C. They also translate Indian languages and Persian. Amazing. So what?
D. Book B is often commented on by new poets. Again, so what? We are talking about those medieval poets aren't we? Why were they not interested?
E. Book B is drama. Group A wrote and performed drama. I feel like crying when I think of how irrelevant this is to our conclusion.
Pick A

~Abhay
Believe those who are seeking the truth. Doubt those who find it. -- Andre Gide