This is a tricky question. The reason it's easy to get this wrong is that answer choice B appeals to our common sense and our real-world context. However, nowhere in the passage is it supported that there would be more frequent clinical trials if we adjusted the ethical standard in place.
The idea here is that "theoretical equipoise" is too strict and an unrealistic ideal. It cannot be met and if it is met, it will almost inevitably fail at some point during the study. The author is suggesting an ethical standard that would be possible to adhere to and allow physicians to complete studies, all the while maintaining their ethical standards.
Perhaps even with this unrealistic notion of theoretical equipoise, there are still a maximal number of clinical trials already occurring. The physicians may simply be going against their ethical standards, because they may complete the studies while having a preference for one treatment or another. Just because we switch ethical standards, does not mean there will be more trials, it only means that physicians will not be able to realistically adhere to the standard.
I hope this helps. Please let me know if you have any other questions.