Proof of insurance must be documented in the form of what?

The correct answer is a Certificate of Insurance (COI) because it is the standard document that provides evidence of insurance coverage. A COI specifically outlines the types of coverage, limits of liability, and the policy period in which the coverage is valid, ensuring that the insurance provider has recognized the insured party's coverage. This document is critical in the context of fire alarm rules and regulations as it demonstrates compliance with legal requirements for insurance in the industry.

Other options do not serve the same purpose. A registration form is mainly used for administrative purposes and does not provide proof of insurance. A certificate of completion typically signifies that a particular training or course has been successfully finished but does not affirm any insurance. An insurance policy may contain detailed information about the coverage but is not universally used as proof; the COI simplifies and formalizes this information for easy verification by third parties.