Linguist: Regional dialects, many of which eventually become distinct languages, are responses by local populations to their own particular communicative needs.
Conclusion (so)
Even if we were to unify the world economy and be forced into adopting a universal language for trade, this language will inevitably develop many regional dialects.
Evidence
Regional dialects are responses by local populations to their own particular communicative needs.
Evaluate
The conclusion has two noteworthy features: 1. it's a conditional (when) 2. there's a new concept (a unified global economy forces the development of a universal language)
Each of these features is suggestive that this will be testing a Missing Link. Generally, when there's a new concept in the conclusion, the correct answer is telling us what the author is assuming about that concept.
Why does the author think this universal language of the global trade economy would inevitably develop many regional dialects?
We have to use the premise, which defines "regional dialects".
The fact that we have an overlapping term that's defined in the evidence also suggests Missing Link.
If I argued ...
... you would know that I'm assuming that "anyone who can do Y can do X".
The new concept in the conclusion 'anyone who can do Y' gets connected to the definition 'can do X", via the overlapping term "blipsy".
In this argument it said, "A regional dialect is a response by local populations to their own particular communicative needs. Therefore, the universal trade language of the future will have regional dialects".
So we know that it's assuming "the universal trade language of the future will have responses by local populations to their own particular communicative needs".
The new concept in the conclusion "universal trade language of the future" gets connected to the definition "responses by local populations to their own particular communicative needs", via the overlapping term "regional dialects".
Goal
Let's look for an answer reinforcing the idea that the author assumes that in this future world where we have a global economy and a universal language used in trade, it will cause there to be local populations who have their own particular communicative needs (leading to new regional dialects).