Thanks for the question! So let’s take a look at what this stimulus is telling us. We’re told that you can tell how quickly a new idea is taking hold among the population based on how fast the word or words expressing that idea are passing into common usage. Notice here, however, that there’s kind of a disjoint between these two ideas. We want to know about a new IDEA, and so we look at the WORDS that express that idea. We’re asked here to find an assumption the argument rests on, and these assumptions might be a little harder to anticipate, but one of them certainly is that there’s some kind of stable link between the words used to express the idea and the idea itself. This happens to be the correct answer: in (D), we’re told that the assumption is that as a word passes into common usage, its meaning doesn’t undergo any severe distortions in the process.
Let’s say that (D) is false, that a word’s meaning does undergo severe distortions in the process of passing into common usage. Then we can definitely say that the word is taking hold among the population, but we can’t say anything about the original underlying idea we wanted to know about, since the word might mean a completely different idea by the time it takes hold in society. Without (D), we can’t determine how quickly a new IDEA is taking hold based on the words, and so (D) is an assumption that the argument requires.
Hope this helps! Feel free to ask any other questions that you might have.
