Before I begin, my background is such: I have not studied for the GMAT yet, only taken the Prep Test #1 (which I scored a 45 q and 40 V). I say this as a disclaimer for any answer I may give. My lack of studies means I am unfamiliar with patterns, tricks, or formal reasoning methods that may be taught in books or understood by you all.

That said, from taking the Prep #1, one thing I noticed is the commonality of the Critical Reasoning questions in referencing what is extraneous information in the problem statement relative to what the question is asking. This problem seems to be one such classic example.

Answers A, B, and C can quickly be eliminated for this reason. All reference a portion of the problem statement that have nothing to do with the method being described (only how its solution can be measured).

D and E remain.

The problem essentially says 'If A (if a new idea takes place) then B (its associated words/connotations will pass into common usage at a similar rate)'. The assumption then, in quantifiying the usage of these words to determine how quickly its associated meaning (or idea) is taking hold is that these words remain constant and are not distorted. If they become distorted but are still being used; then you will get an inaccurate measure of how much the idea is taking hold. Simply put, the method no longer works. The answer is B.

But why not E? (This likely most picked wrong answer). E is tempting because it deal with the method (not how to measure its solution) like B does. But look closely; the question asks what assumption is made for the method described to hold water; E would be an assumption NOT made by the author as it is anathema to his theory. If words denoting new ideas were used before they ideas are understood; his solution for measuring the suffusion of a new idea would not be true.

Yup, answer is D. Hope I'm right.

Argument says:--

-->quicker the word/words expressing an idea becomes popular, faster that idea becomes popular.

-->Dictionary editiors are also concerned with above written finding who have methods to know whether or not a word is passed into common usgae.

Now say a word became popular over a period of time. Dictionary Editors found it really popular using their techniques. Can you say that idea represented by that word is is also equally popular?

Not necessarily. A new word may hold mind of public for its novelty and idea at the time it was introduced, but over a period of time, it may loose its original meaning , however it can still be popular and being used in other contexts.

Therefore, there is one clear assumption in above written argument:-
"As a word passes into common usages, its meaning does not undergo any severe distortions in the process."

D wins.
A and B --> Nothing to do with popularity of an Idea
C -->Dictionary Editors only measure common usage of word not popularity of an Idea
E -->Close enough. Word may be popular before, but it is not necessary that idea will be understood later.It may be, it may not be.

As a matter of fact, there are many such English words in the Dictionary which are being used in different context today.

choice D stands.

if this is it,then do a negate test.if the assumption is negated ,then the conclusion falls apart.
As a word passes into common usages, its meaning does undergo any severe distortions in the process.hence it means that the original meaning of the idea is now distorted.
conclusion falls aprt.

D...