B. A research study conducted by the Mayo Clinic
This option is likely to provide the most bias-free information because the Mayo Clinic is a reputable medical institution known for its evidence-based research and focus on patient care. The other options are more likely to present bias due to their associations: option A comes from patients who may have personal stakes or experiences influencing their perspectives; option C is sponsored by tobacco companies, which have a vested interest in downplaying health risks related to smoking; and option D, being conducted by attorneys, may have a legal angle that could affect objectivity.