Final answer:
A carrier must keep an accident register for 3 years in accordance with FMCSA regulations to maintain a proper record of all accidents involving their commercial motor vehicles.
Step-by-step explanation:
According to the Federal Motor Carrier Safety Administration (FMCSA) regulations, the correct answer is B) 3 years. A carrier must retain an accident register for each accident involving its commercial motor vehicles for a period of 3 years after the date of the accident. This regulation helps ensure that a proper record of accidents is maintained, which can be crucial for safety audits, compliance reviews, and litigation that might occur after an incident.