The RC is posted below for your reference.

The Q is :
It can be inferred that the author makes which of the following assumptions about biographies?
A.Their main purpose is to inform readers about key aspects of the subjects' personalities. - There are enough instances to suggest that this was the purpose. From selecting the right biographer, to getting the right material, to analyzing whether a compelling book can be written. The underlying emphasis is on informing.

B.Only subjects who share traits with biographers make good subjects for biographies. - Contrary is mentioned.

C.Compelling biographies cannot be written about ordinary citizens. - Extreme, The passage says that compelling biographies require characteristic personality traits. To confuse us "ordinary citizens" is used which seems close to ordinary personalities.

D.The biographer's credibility with readers is a factor in the critical success of a biography.
Your explanation:
(D) CORRECT. In the second paragraph, the author discusses the elements of a good biography, stating that “a biographer’s knowledge and ability also determine the choice” of subject. If the author did not assume that the biographer's credibility with readers is a factor in the critical success of a biography, then this part of the second paragraph would be meaningless. - ------ The credibility bit only defines the choice of subject. It does not state that readers use this as a parameter. Nowhere does the reader come in, except for in the last para, where it is mentioned that the writing a good book for the reader is critical.

E.Practical considerations are most important in the selection of a subject for a biography. - Out. Main point is to write a good book

My analysis is as stated above and I am stuck with the two choices in Bold. I cant agree with your test explanation.
Please provide your response.

One often hears that biographies are autobiographies, that the biographer is always writing about himself. On the contrary, serious biographers seek and welcome the unfamiliar, however troublesome to account for. Ron Chernow, the author of rich biographies of the American businessmen J.P. Morgan and John D. Rockefeller, remarks that biographers "like to stub their toes on hard, uncomfortable facts strewn in their paths." Such encounters with the unaccountable are opportunities for breaking out and breaking through, in new directions, to fresh understanding.
One also often hears that biographers must like their subjects. That would of course rule out such vastly important subjects as Hitler or Stalin. In practice, the biographer must like the subject not as a person but as a subject. Some are good subjects for the author, some bad. And what makes one subject better than another for any particular biographer varies dramatically. Some of the reasons are purely practical. Does the subject need a biography? Are the materials available? How much time is needed? A biographer's knowledge and ability also determine the choice. Great scientists are great subjects, but can one write about their achievements with insight and authority? Personal idiosyncrasies matter, too. Biographers tend to be attracted to subjects who display particular personality traits, whether they be ambition, cruelty, ingenuity, or any other characteristic that separates a potential subject from the multitudes.
In choosing a subject, the biographer's main question should be, "Can an effective book be made out of this person's life?" Day after day for years, the biographer will try to untangle chronology, compress relationships without distorting them, and keep the main narrative clear while carrying forward several intricate strands of the subject's life. What pushes most biographers on in this endeavor is not necessarily affection for the subject but the feeling that they are writing a good book.

Not ignoring you! I've got to look something up on this one.

Sorry for the delay - had to talk to our curriculum director for this response. I agree - I think D is too much of a stretch and is not sufficiently GMAT-like, and he had actually already flagged this one, too. We run statistical analyses on our questions as our students answer them and either re-write or discard questions that are not predictive enough (which means they have to hit a bunch of parameters that have to do with validity, consistency, etc.), and this one has recently come up as problematic. It's in the midst of being rewritten.

By the way - answer A is also not GMAT-like. Neither one really infers in the way in which the GMAT infers. So it isn't that A is a better answer - basically, there isn't an answer I like on this one.

Thanks for that clarification.
If, let's say, on the actual test I was forced to select a choice, it would have to be A..
Nowhere does D fit in.
As per the "point system" you use for the main idea: D will surely score a low second.

Samy,

Good question. We are always learning from the feedback our students and forum users provide. Thanks!

-dan

IMO it is E

A) The passage says that the biographer should like the subject and not their personal traits (Hitler and Stalin)

B) No such mention in the paragraph

C) Nothing is mentioned of ordinary citizens

D) Credibility of the biographer is never talked about.

E) This is correct since the author mentions about practical considerations.

"What makes one subject better than another for any particular biographer varies dramatically. Some of the reasons are purely practical."

This one has since been re-written - don't study from the old version b/c it's problematic. :)