The success of the program to eradicate smallpox has stimulated experts to pursue what they had not previously considered possible - better control, if not eradication, of the other infections such as measles and yaws.

a) what they had not previously considered possible - better control, if not eradication, of the other infections such as

b) what they had not previously considered a possibility - better control, if not eradication, of such infections like

c) something they had not previously considered possible - better control, if not eradication, of such infections as

d) something not considered a previous possibility - better control and perhaps eradication, of other infections such as

e) the possibility of what they had not previously considered - better control and possibly eradication of infections like

Source GMAT Prep

What's the Key difference between A and C?

Is "other" in A or the TENSE=HAD

What's the role of What..."what they had not previously considered possible

Is this what kind of Construction generally wrong on the GMAT?

I have seen many questions that have this "WHAT ...." Construction as wrong answer.

Please Explain

when you're tabulating wrong answers, never, never forget that 80 percent of all answer choices are wrong. therefore, if you simply observe grammatical patterns that occur in wrong answers, you might wrongly conclude that just about everything is 80 percent incorrect.

nothing is fundamentally wrong with the 'what' construction you're mentioning. in some sentences, it's among the best options, if not the best option outright:
the reality of small-town life was nothing like what i had pictured.
there are other ways to structure this sentence, but this way is certainly correct, and is one of the best and most concise ways to phrase it.

however, the 'what' construction is problematic in this sentence, because, in general, 'what' conveys a sense of exclusivity / uniqueness. 'what they had not considered possible' implies that there is only one thing satisfying that description, in contrast to 'something...' which admits the possibility of many other such things.

if the above is not clear, consider the difference between the meanings of the following two sentences:
a big family is what i've always wanted. --> this has been my primary goal in life, if not my only goal in life
a big family is something i've always wanted. --> in addition to a nice fast car, hundreds of leather-bound books, and a refrigerator full of finnish beer.

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the biggest bugbear in choice a is the word 'the', which creates an image of a definite set of other infections. consider these two sentences as an analogy:
our son has always chosen football over the other sports. --> implication: our son is choosing football from a known list of options
our son has always chosen football over other sports. --> implication: our son is choosing football over any other sport that could possibly exist

choice c avoids this problem by saying simply 'such infections as'.

What is the issue with "D"? What is grammatical error in this ans choice?

Apologies for restarting the thread.

the first part - 'something not considered a previous possibility' - has a substantively different meaning from the original, in addition to being generally awkward. in particular, 'previous possibility' carries a meaning roughly equivalent to 'was a possibility before, but isn't anymore'. that is not what is meant.

there also needs to be a comma between 'control' and 'and', but that might just be a transcription error (as the gmat very rarely picks on punctuation).

"had not previously"

Isn't this redundant... as we can easily say "had" implies past ...

Please explain
Anon

apparently not.

we could argue this issue for a while, but it's clear that the standards set by the gmat test writers allow this expression (as proved beyond a doubt by this problem). so, regardless of whether it seems redundant to you, file it away in your head as an acceptable expression.

it definitely serves to create a clear demarcation between the former situation (not considered possible) and the subsequent situation (considered possible); that distinction would be nowhere near as clear without the 'previously'.
but at the end of the day, the only thing that really matters is that the gmat uses this expression, so you can (and should) too.