The correct answer is C) A biography of a presidential candidate written by the candidate's main opponent.
The source that is the most likely to contain a problematic bias is "A biography of a presidential candidate written by the candidate's main opponent."
The rival candidate could find hard to maintain objectivity when writing the biography because he was his political rival and it is probable that he could have some resentments and differences that could bias the information of the book. It would be hard for him to write objectively and readers should read the information carefully, knowing that what is expressed in the biography could be biased and needed other sources to confirm its validity.
