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:
Option 1.
If he dies before he has received in payments the present value of his retirement allowance as it was at the time of his retirement, the balance shall be paid to his legal representatives or to such person as he shall nominate by written designation duly acknowledged and filed with the board of trustees; or
Option 2.
Upon his death, his retirement allowance shall be continued throughout the life of and paid to such person as he shall nominate by written designation duly acknowledged and filed with the board of trustees at the time of his retirement; or
Option 3.
Upon his death, one-half of his retirement allowance shall be continued throughout the life of and paid to such person as he shall nominate by written designation duly acknowledged and filed with the board of trustees at the time of his retirement; or
Option 4.
Some other benefit or benefits shall be paid either to the member or to such person or persons as he shall nominate provided such other benefit or benefits, together with the lesser retirement allowance, shall be certified by the actuary to be of equivalent actuarial value to his retirement allowance and shall be approved by the board of trustees.
§ 86.100, RSMo