Recent research shows that sound change (pronunciation shift) in a language is not gradual.
Conclusion
Sound-change theory in general must be discarded.
Intermediate Conclusion (since)
The classical account must be discarded.
Evidence
The classical account's central tenet was that sound change occurs gradually, but recent research shows that sound change isn't gradual; it often emerges suddenly.
Evaluate
This question stem looks like Sufficient Assumption because of the "if assumed" but it's only a Strengthen question because it just asks what does the most to justify the conclusion.
We can still use our Sufficient Assumption brain (heavy on conditional logic), but we want to appreciate that the correct answer might come more from the conversational world of Strengthen.
Using our conditional logic brain, we'd think 1. We need to prove "sound-change in general must be discarded" 2. They haven't announced any rule like, "If xyz is true, then must be discarded", so the correct answer will need to give me that rule. 3. The reason for discarding it, in general, is that the classical account must be discarded.
That would lead us to anticipating a missing link answer like: .
Using our Strengthen brain, we'd think,
An opponent could say, classical account is basically 90% of sound-change theory in general, so if you get rid of classical, you are getting rid of the majority of sound-change theory".
Goal
Let's find an answer that strengthens the connection between the premise and the Intermediate Conclusion (if recent research shows that new sounds often emerge suddenly, then classical's central tenet is ruined and thus classical should be discarded).
Or let's find an answer that strengthens the connection between the Intermediate Conclusion and the Main Conclusion (if we're getting rid of the classical account, we should just get rid of sound-change theory in general).