Final answer:
TRUE. The insurer must provide a written explanation for denying or rejecting a claim, and the claimant can request a review from the California Department of Insurance.
Step-by-step explanation:
When an insurer denies or rejects an insured's claim, in whole or in part, it is required to do so in writing and include the basis for the rejection or denial. If a claimant believes that their claim has been wrongfully denied, they can request a review from the California Department of Insurance. The insurer must inform the claimant of this option and provide contact information for the department unit that reviews claim practices.