The passage starts out by telling us about a belief that some legal scholars hold regarding defence lawyers' obligations to their clients.
Starting in the second paragraph, the author presents their own view of defence lawyers' obligations which they argue are to the court and to society.
The author argues that defence lawyers should not present assertions to the court that they know are false and that guilty defendants should not be entitled to false or insincere representation. If a defence lawyer knows that their client is guilty, then they must not obscure this fact, working instead to present any extenuating circumstances and arguing for leniency in sentencing to the extent that they genuinely believe is warranted.
The author argues that, if a lawyer sincerely believes their client may well be innocent, then they should try to prove this.
The author concludes the passage by holding that defence lawyers following their obligations to the court and to society also benefits the defendant. Why? Because lawyers upholding their obligations provides the "best defence," allowing lawyers to be advocates for their client's rights given the facts of the case.
All of this is restated by answer choice (B) which tells us that the primary purpose of the passage is to argue that the defence lawyer has obligations to the court and to society and that following these obligations provides their client with the "best defence," or effective legal representation.
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