Why A and not B?

The term integrity is used to show that at least some of the transcribers were reliable, thus we would not have any real concerns as to the biographical details they recorded. However, A directly is used to cast doubt as to the authenticity of the stories. Saying something is ostensibly biographical is directly casting doubt on the claim that it is actually biological.

But the question says "best serves to 'underscore' the author's concerns about the authenticity of the autobiographies discussed" I assumed that meant to weaken, in a way, the fact that the author thinks that these editor based books are compromised.

Underscore means to emphasize or highlight. Thus, it would emphasize the idea that these narrators are not necessarily reliable.