OE:
Knowing this not too uncommon idiom is helpful. “Just as” will be followed by an independent clause (putting together “just as” with the independent clause will make it a dependent clause). At the end of that clause, there will be a comma. Immediately following that comma should be a “so” (the “too” is optional).
What’s important is that the part after “just as” and the part after “so(too)” should be parallel.
We can eliminate answer choices (A) and (C) because they do not follow the idiom.
(A) See above. Otherwise, this answer is grammatically correct.
(B) This answer implies that today’s immunologists use the pustules from milkmaids. Remember, the parallelism of the two clauses in the “just as…so too”: Just as Jenner used pustules from milkmaids…so too do today’s immunologists. The parallelism, both grammatically and semantically, is between Edward Jenner using pustules from milkmaids and today’s immunologists using pustules from infected organisms.
(C) See above.
(D) The parallelism between Jenner and today’s immunologists is maintained. The arrangement of the sentence maintains the meaning of the original: “the live virus from which the modern smallpox vaccine is derived.”
(E) implies that the smallpox vaccine is derived from the pustules of infected organisms, because of the use of “which”. Technically, there are three noun subjects “organisms”, “pustules” and “live virus”. Based on the “touch rule”, which “which” adheres to, “which” should refer to the first subject in a prepositional phrase. But with one as long as this, it becomes a little confusing just what “which” refers to. The correct answer in the GMAT would find a clearly way of expressing the fact that it is the vaccine is obtained from the “live virus”. Finally, “obtained FROM”, not “obtained IN”, is the correct idiom.