One good way to attack the question is by negating the assumption. If we do so with (E) we get:
Negated Answer Choice: The Plague causes genetic mutations such as Delta-32.
If the Plague indeed caused Delta-32 to occur, then the researches hypothesis totally falls apart: Delta-32 did not protect against the plague.
Answer choice (C) says that Delta-32 came into being around the 16th century, about the time of the Plague. Therefore, it could have conceivably conferred immunity on those who carried the gene.
Hope that helps
Magoosh Test Prep