Final answer:
The Medical records are crucial for an underwriter to assess an applicant's health status and treatment history for insurance risk assessment. Therefore, The correct answer is Option A) Medical records.
Step-by-step explanation:
An underwriter reviewing an applicant with an extensive medical history will gain the most comprehensive understanding of the insured's treatment for various illnesses by examining the applicant's medical records. Medical records provide detailed information about past diagnoses, treatments, and outcomes, which are crucial for the underwriter in assessing the insurance risk. Other options like driving records, credit reports, or employment history do not offer information about an individual's health status or medical treatments.