(a) Every insurer shall maintain an unearned premium reserve on all policies in force for:
(1) Insurance against loss or damage to property, except as provided in section 431:5-302;
(2) General casualty insurance;
(3) Accident and health or sickness insurance, except as provided in section 431:5-303 and section 431:5-307; and
(4) Surety insurance.
(b) For purposes of this article, "unearned premium reserve" means the portions of the gross premiums in force, less authorized reinsurance.
(c) All reserves may be computed, at the insurer's option, on a monthly or more frequent, pro rata basis.
(d) After adopting any one of the methods for computing such reserve, an insurer shall not change methods without the commissioner's approval.