This question is rich in splits!
Modifier/meaning:
previous possibility vs.
previously considered.
The experts' thinking about the possibilities has changed, not the possibilities themselves. Eliminate (D) for
previous possibility.
Comparison or Example?:
such as vs.
like vs.
such...likeOn the GMAT,
like means one thing: similar to. Since measles and yaws are examples, use
such as to introduce them.
Such X like Y is unidiomatic. Eliminate B and E.
Wordiness:
the other vs. (nothing)
By presenting other diseases as examples using
such as, it is clear they are not the same as smallpox. Also, the three diseases mentioned have three different names. Using the word "other" is unnecessary, and prefacing it with the definite article "the" is weird in this context. Eliminate A.
That leaves C as the answer, but A and C are pretty close... The other split between A and C is
what vs.
something. Either could be the beginning of the noun phrase that serves as the object of
to pursue, but the GMAT uses
what in this capacity more often, so
something is just a bit awkward, relatively speaking.