A number of linguists contend that all of the thousands of languages

I'm happy to help with this.

First of all, on the GMAT, verb like contend, argue, suggest, think, assert, conclude, etc. require a "that"-clause, and the word "that" must appear. In colloquial English, I can say "I thought he was coming" --- that would be fine in ordinary conversation, but it is not up to GMAT standards. GMAT grammar standards demand the word "that" in this context.

Here, that immediately eliminates (C) & (D) & (E). Now, we just choosing between (A) & (B).

Both (A) & (B) are grammatically correct, and it is more a question of emphasis. The substance of the linguists' claim is that the 1000s of languages have a common root language --- a mind-blowing idea. How many people are on earth speaking these 1000s of languages is more a detail for emphasis, and should not be front & center. Choice (A) makes the "thousands of languages ... can be traced" the main subject & verb of the "that"-clause, and this is the heart of the linguist claim. Choice B emphasizes the words "the world’s five billion people speak" by making that the main subject & verb of the "that"-clause ---- this makes brings something that should be a detail to front & center, and relegates what should be the main focuses to a secondary subordinate clause nested inside the "that"-clause. In choice B, the grammatical relations are at odds with the logical priorities. In a well designed sentence, grammar should support logic and the priority of the argument at all steps. Therefore, (B) has problems, but (A) is the best possible answer.

Arguably, choice A is not ideal --- arguably, one could find a more elegant way to express this information. Nevertheless, on GMAT SC, the job is not to find the best possible sentence, but only to find the best among the five available options. Here, (A) is the best of the five.

Does all this make sense?

Mike