The correct answer is A.
Answer A says that Elizabethans had at least 23 medical treatises, but none of them were helpful in fighting the plague.
This means that the full extent of Elizabethan medical knowledge was not helpful in surviving the plague. It is not surprising, then, that the plague took such a toll on them.
Therefore, Answer A best develops the central idea that the plague was almost impossible for Elizabethans to survive.