Mo. Rev. Stat. § 86.100

With the provision that no optional selection shall be effective in case a beneficiary dies within thirty days after retirement, in which event such a beneficiary shall be considered as an active member at the time of death. Until the first payment on account of any benefit becomes normally due, any beneficiary may elect to receive his benefit in a retirement allowance payable throughout life or he may elect to receive the actuarial equivalent at that time, of his retirement allowance in a lesser retirement allowance, payable throughout life with the provision that: