One sure way you can tell how quickly a new idea—for example, the idea

The answer is D


This is a Necessary Assumption question. So, we're looking for an answer that is required to be true, in order for the conclusion to have a chance of being true.

The conclusion of the argument is that one can tell how quickly a new idea is taking hold by by monitoring how how fast the word is passing into common usage. The problem with this is that the idea may no longer be represented in the word by the time it becomes common. The meaning of the word could have shifted and so the idea is not taking hold in society, even though the word is. This is best expressed in answer choice (D). Negating answer choice (D) would say, "As the word passes into common usage, it's meaning does undergo severe distortions in the process." The meaning of the word would no longer be the same, so tracking the word, would not be tracking the idea.

(A) is not necessary. They could be interested in words that are rarely used, but that doesn't mean that the editors would in their professional opinions claim that the words had passed into common usage.
(B) is not necessary. How the editors determine whether the word has passed into common usage is not important.
(C) is backwards. it's not that the editors must include the word in their dictionaries for the word to take hold, but that once it takes hold, it gets included in the dictionary.
(D) is necessary to for using the common usage of a word to determine whether a new idea is taking hold in society.
(E) would tend to weaken the argument, so is not necessary to it. If the words are being used before the meaning is understood, then tracking the word tells us nothing of whether the idea is taking hold. People would be using the word without an understanding of its meaning.