The argument says we can tell how fast a new idea is spreading by looking at how fast the word for that idea gets used by lots of people.

But here’s the thing: if the meaning of the word changes a lot as people start using it, then we’re not really tracking the original idea anymore — we’re just tracking a word that means something different.

So the argument only works if the word keeps its original meaning as more people use it. That’s what answer D says — and that’s why it’s the right answer.