This theory has gradually won acceptance in the scientific community
If the theory has won acceptance, that strongly suggests that more than half of the scientific community accepts it. Less than that wouldn't be much of a win! More than half is enough to say "widely".
Consider this, if you're not convinced: if the theory was not widely accepted, would the author have used these words? Or would he instead have said something more like "has begun to find some support" or "is starting to find some acceptance"?
It may be hard to say how much acceptance is "wide", but one last thing to consider here is that there is no better answer choice, and we are supposed to pick the best one. It's not hard to challenge answer D here as being less than perfect, but that's not the standard we should apply to answer choices on this test. The best answer, even if you don't love it, is still the best one!
Adam M. Tyson
PowerScore LSAT, GRE, ACT and SAT Instructor
Follow me on Twitter at https://twitter.com/LSATadam